Honors Chemistry
Unit 1 - Introduction to Chemistry's Measurements
and Problem Solving
Unit 1: Introduction to Chemistry’s Measurement and
Problem Solving
Part 1 – Notes: Identifying Significant Figures
Objectives: Identify the number of significant
figures in a measurement.
Compare relative uncertainties of
different measurements.
Text Reference: Section 3.2 (part) – pages
54–58(top)
Any measurement involves an estimate and that
means that there is some uncertainty in that measurement. When a
measurement is recorded, it includes all the digits that are certain plus
the single estimated digit. These certain digits plus one uncertain
(estimated) digit are referred to as significant figures. The
certainty of a particular measurement is indicated by the number of significant
figures recorded in that measurement.
***KEY POINT*** The further the
estimated digit lies to the right in a measurement, the less relative
uncertainty there is, so the more reliable the measurement.
Look at the measurements:
34.3 mL 34 mL
34.46 mL 34.458 mL
Which of the measurements has the least amount of relative
uncertainty?
Which of the measurements is the least certain (least
reliable)?
Rules for Identifying Significant Figures
1. Nonzero Integers. Nonzero
integers always count as significant figures. For example, the
measurement 1479 m has 4 nonzero integers, all of which count as significant
figures. 1479 m = 4 s.f.
2. Zeros. There are three
classes of zeros:
a. Leading zeros are zeros that precede
all of the nonzero digits. These leading zeros never count
as significant figures. For example, in the measurement 0.0025
g, the three zeros simply indicate the position of the decimal point.
The measurement has only two s. f., the 2 and the 5.
b. Captive zeros are zeros that fall between
nonzero digits. They always count as significant digits. For
example, the measurement 4.005 L has four significant figures.
c. Trailing zeros are zeros at the right
end of the number. They are significant only if the number is written
with a decimal point. For example, the measurement 3.5000 g has
a decimal point and the trailing zeros are significant; the measurement
contains 5 s. f. The measurement 100. m is written with a decimal
and contains 3 s.f while the measurement 100 m does not have a decimal
and therefore contains only 1 s.f. (Note, a bar may also be placed
over a zero to indicate that it is significant.)
3. Exact numbers. Often calculations
involve numbers that were not obtained using measuring devices but were
determined by counting: 7 beakers, 3 apples, 8 molecules. Such numbers
are called exact numbers. They can be assumed to have an unlimited
number of significant figures. Exact number may also arise from definitions.
For example, 1 inch is defined as 2.54 cm; neither 2.54 or 1 limits the
number of significant figures when it is used in calculations.
Examples: How many significant
figures in the following measurements and where is the uncertainty?
6072 m
0.004 040 g
6070 L 6070.
L 3000. g
0.004 04 L
49 500 m 3000 m
372.00400 L
0.000000004 m
Unit 1: Introduction to Chemistry’s
Measurement and Problem Solving
Part 1 – Assignment: Identifying Significant Figures
Indicate the number of significant figures in
each measurement. Then circle the uncertain digit.
1. 679.98 g
2. 50 002 L
3. 4.000
m
4. 0.000
9 m
5. 3001 g
6. 3000
m
7. 0.000 400 m
8.
0.000 06 g
9. 10 001 m
10. 10 000 L
11. 100.001 g
12. 0.010 10 mg
13. 0.809 00 nm
14. 10 030 L
15. 0.000 300 4 dam
16. 34 578 pg
17. 40 908 mL
18. 5.80 hL
19. 0.009 00 km
20.
3000. kg
Unit
1: Introduction to Chemistry’s Measurement and Problem Solving
Part 2 – Notes: Mathematical Operations and Significant Figures
– 1
Objectives: Correctly record an answer
to a single-step math problem with the appropriate number of significant
figures.
Determine, utilize, and explain
the rule for the number of significant figures in a sum or a difference.
Determine, utilize, and explain
the rule for the number of significant figures in a product or quotient.
Explain how the rules for determining
the number of significant figures in single-step math problems are derived.
Text Reference: Section 3.2 – pages 54-62
Adding and Subtracting Significant Figures
When adding or subtracting measurements, the sum or difference
can only be as certain as the least certain measurement.
Remember, properly recorded measurements are recorded using significant
figures, all certain digits plus a single uncertain estimated digit.
Example 1: Add the following and report
the answers to the correct number of significant figures.
(A) 23.0042 m
+ 9.0 m (B) 12.7 g + 3.3 g
(C) 19.38 cm + 2.4 cm
(D) 9.1 m + 11.01 m + 10
m
The position of the uncertain digit with relation to the decimal
point in the least certain measurement determined the position of the
uncertain digit with relation to the decimal point in the answer.
For our purposes, the uncertain digit should be rounded up or rounded down,
just as in math.
In your own words, what’s the rule for adding and subtracting?
Multiplying and Dividing Significant Figures
When multiplying and dividing measurements, the answer
may only contain as many significant figures as does the measurement
used with the least amount of significant figures.
Example 2: Find the products of the following
and report the answers to the correct number of significant figures.
(A) 42.0 m x 4.0 m
(B) 1.78 cm x 0.05 cm
(C) 1001 mm x 40 mm
(D) 1001 mm x 40. mm
Note that unlike addition and subtraction, the position of the
uncertain digit with relation to the decimal point in the answer does
not have to be in the same position as the position of the uncertain digit
in any measurement.
In your own words, what’s the rule for multiplying and dividing?
Unit 1: Introduction to Chemistry’s
Measurement and Problem Solving
Part 2 – Assignment: Mathematical Operations and Significant Figures
– 1
Solve the following.
Show all work and intermediate steps and record the answers to the correct
number of sig figs.
1. 23.67 m + 45.7 m
2. 67.2 g – 30.4 g
3. 56 m x 20.4 m
4. 1020 m ÷ 20 s
5. 10 000.0 m + 1 m
6. 10 000 m + 1 m
7. 1020 m ÷
20.0 s
8. 5.67 m + 10.0 m
9. 5.67 m + 10 m
10. 1000 g + 5 g
11. 15 m x 5 m
12. 15 m x 5.0 m
13. 3500 m2 ÷
700 m
14. 300 g – 6 g
15. 4000.9 g + 1 g
16. 4500.5 m – 0.5 m
17. Water with a mass of 35.4 g is added to an
empty flask with a mass of 87.432 g. The mass of the flask and
the water is 146.72g after a rubber stopper is added. Express the
mass of the stopper to the correct number of significant figures.
18. Criticize this statement: “When two measurements
are added together, the answer can have no more significant figures
than the measurement with the least number of significant figures.”
Unit
1: Introduction to Chemistry’s Measurement and Problem Solving
Part 3 – Notes: Mathematical Operations and Significant Figures
– 2
Objectives: Correctly record an answer
to multiple-step math problems, averages, and conversions with the appropriate
number of significant figures.
Additional objective – see
objectives from Unit 1 Part 2.
Text Reference: Section 3.2 – pages 54-62
Rule 1: Multiplication, Division,
and Exponent Operations, only:
When multiplying, dividing, and using exponents only, perform all
operations first and then apply the rule for significant figures to the
answer.
Example 1: (2.48 m)2 (2.58
m) ÷ 5.78 m
Rule 2: Addition and/or Subtraction
in combination with other operations:
When addition or subtraction operations are used in conjunction
with other operations, follow the algebraic order of operations, applying
the rules of significant figures after addition/subtraction is completed.
Then perform the multiplication/division as necessary and then apply
the rules for significant figures, again.
Example 2: (4.238 m + 5.97 m) ÷
2.5 s
Example 3: [(4.56 m + 5.679 m) (2.00
m + 9 m)] ÷ 27.9 m
Rule 3: Numbers with NO uncertainty:
All conversion fractions, constants, and numbers with no uncertainty
are treated as if they have an infinite number of significant figures.
More on this part later.
Example 4: Convert 14.75 hours to seconds.
Rule 4: Averages and uncertainty:
When finding the average (mean) of measurements, the mean cannot
be more certain than the least certain measurement. Also, keep
in mind that you are adding and dividing (two different significant figure
rules).
Example 5: Calculate the mean of the
following measurements: 2.784 g and 9.863 g.
Unit 1: Introduction to Chemistry’s
Measurement and Problem Solving
Part 3 – Assignment: Mathematical Operations and Significant Figures
– 2
Perform the following operations
and record your answers to the correct number of sig figs. Show
all intermediates.
1. 46.98 m ÷
(3.5 s x 5.78 s)
2. (56.7 s + 12 s) (35.46 s)
3. (5678.1 m x 2.0 m) ÷
5 m
4. (12.3 m + 15 m) ÷
2 s
5. 43400 m3 ÷
(4.334 m + 44.0002 m – 0.982 m)
6. Convert 5 789 seconds to days.
7. Average: 5.0 g and 7.89 g
8. Average:
9.1 g, 11.0 g, and 10 g
9. Average: 2.345 g and 8.349 g
10. Average: 5.00 g, 55.0 g,
and 200 g
Unit
1: Introduction to Chemistry’s Measurement and Problem Solving
Part 4 – Notes: Scientific Notation
Objectives: Convert a number in standard
notation to correct scientific notation, and vice versa.
Identify the part of a number
in scientific notation and state appropriate values for each part.
Distinguish between qualitative
and quantitative measurements.
Perform simple math problems
with number in scientific notation without a scientific calculator.
Text Reference: Section 3.1 – pages 51-53
Qualitative:
Quantitative:
Measurements in chemistry range from incredibly tiny to almost
unimaginably large. The number of iron atoms that would fit side
by side on a line 1 cm in length is more than 80 million. The number
that could be packed into a cube with a volume of 1 cm3 is
(80 million)3 – or about 500 thousand billion billion!
The size of an iron atom is almost unimaginably small.
Scientists use large and small numbers frequently, but with so
many zeros, problems are frequently encountered. Writing large
numbers of zeros is time consuming and there is an increased chance of
an error being made when writing many zeros. Scientists handle
large and small numbers using scientific notation, also called
standard exponential form. A number written in scientific
notation has the following appearance and parts:
a.bc x 10d
Part
|
What is It?
|
Acceptable Values
|
a.bc
|
|
|
x 10
|
|
|
d
|
|
|
A positive exponent indicates. . .
A negative exponent indicates . .
Converting a number form standard form to scientific notation:
1. Write
the decimal point directly to the right of the first nonzero digit,
creating a number that is greater than or equal to one but is less than
ten.
2. Count
how many places from your new decimal to the original decimal place.
This becomes the exponent.
3. If the
number was larger than one, it remains a positive exponent. If the
number was smaller than one, it becomes a negative exponent.
Keep in mind, this is not magic and it is not something new and
different. All you have done is rewrite a number is different form!!!
Example 1: Write the following numbers
in scientific notation.
(a) 12340000000000000
(b) 45060000000000
(c) 0.00000000000000000234 (d)
0.000000000002000015
Example 2: Convert the following from
scientific notation to standard notation.
(a) 2.36x104
(b) 2.654498x103
(c)
3.659x10-7
(d)
6.019x10-3
Changing the Form of Exponential Numbers
You can increase either the coefficient or the exponent
of a number scientific notation by any factor without changing the overall
value of the number as long as we reduce the other
portion by the same factor. In other words, as one part increases,
the other part decreases. (Remember: Elevator go up, elevator go
down.)
Example 3: Change the following to
correct scientific notation:
(a) 10x107
(b) 0.0050x106
(c) 303x10-4
(d) 0.012x10-7
Example 4: Change the following
coefficients so they match the exponents that are given.
(a) 4.98x106
= _______________x103
(b)
5.686x108 = _______________x1010
(c) 6.37x10-4
= _______________x10-7
(d) 3.587x10-7
= _______________x10-5
Multiplication and Division with Scientific Notation
To multiply numbers in scientific notation, you need to multiply
the coefficient portions and the exponent portions separately.
To multiply the exponential portions, the exponents are algebraically
added. (Be aware of signs!) For example:
(2.0x103) x
(4.0x102) =
To divide number in scientific notation, you divide the coefficients
portions and the exponent portions separately. To divide the exponential
portions, the exponents are algebraically subtracted.
(Be aware of the signs!) For example:
(6.0x105) ÷
(2.0x103) =
Often, when dividing and multiplying, the product or quotient will
be a number greater than or equal to ten or less than one. When
this happens, you need to adjust the coefficient so it is a number greater
than or equal to 1 but less than 10. You then need to adjust the
exponent so the value of the answer is not changed. Your final answer
always needs to be in proper scientific notation.
Addition and Subtraction with Scientific Notation
When we add or subtract numbers in exponential notation, the exponents
must be the same. This rule is the same as making sure the decimals
of your numbers you add or subtract are aligned. The answer is
the sum or difference of the coefficients with the same exponent as each
number in the problem. Remember, the final answer needs to be in
correct scientific notation form.
Example 5: Perform the following
operations.
(a)
(3.0x106) (5.0x104)
=
(b) (4.25x10-5)
(7.7x102) =
(c)
(5.0x103) ÷
(6.75x105) =
(d) (2.5x10-5) ÷
(5.6x10-2) =
(e)
(3.45x106) + (4.56x107)
=
(f)
(2.45x10-4) + (3.75x10-2)
=
(g)
(2.98x105) – (3.25x104)
=
(h)
(9.68x10-3) – (4.67x10-4)
=
Unit 1: Introduction to Chemistry’s
Measurement and Problem Solving
Part 4 – Assignment: Scientific Notation
1. Which of the following are NOT in proper scientific
notation:
a.
2.3x10-4
b. 45.7x106 c.
0.23x103 d.
1.234x10-3
e.
0.004x10-9 f. 1x108
g. 12.3x106
h. 0.48x10-7
2. Write the following numbers in correct scientific
notation.
a. 10100000000000000000000000000
b.
120
c. 0.00000000000987
d.
0.00000000005600007
e. 6.87
f. one billion
g. 0.00000090
h. 700200000
Perform the following operations without a scientific calculator.
Show your work, any intermediate steps, and clearly indicate your final
answer.
3. (6.883x10-4)
(7.883x10-7) =
4. (3.9857x107) ÷
(7.395x1011) =
5. (4.23x108) ÷
(7.65x10-3) =
6. 7.993x10-8
+ 8.883x10-10 =
7. 8.997x10-7
– 9.987x10-6 =
8. 7.856x106
+ 9.546x108 =
9. Identify the following as qualitative or quantitative:
a. A flame
is hot.
b. A candle has a mass of 90
g.
c. Wax
is soft.
d. A candle’s
height decreases 4.2 cm/hour.
10. Rank these numbers from smallest to largest:
(a) 5.3x104
(b) 57x103
(c) 4.9x10-2
(d) 0.0057
(e) 5.1x10-3
(f) 0.0072x102
__________ __________
__________ __________ __________
__________
smallest
Unit
1: Introduction to Chemistry’s Measurement and Problem Solving
Part 5 – Class: Let’s Start Problem Solving
Objectives: Develop and utilize a method
for problem solving.
Text Reference: Section 4.1 – pages 83-88
How would you solve the following problems? Show ALL work,
steps, units, etc.
1. A car travels 270 miles in 5 hours.
How many hours will it take the car to go 150 miles. (Assume it
travels at a constant velocity.)
2. Convert 3000 meters into kilometers.
3. The density of a particular material is 2.3
g/cm3. What is the volume
of 800 g of this material?
4. Let x = number of meters and y = number of
kilometers. An equation using the symbols x and y and the number 1000
which expresses the relationship between the number of meters and the number
of kilometers is:
(A)
1000x = y (B)
1000y = x (C)
x + y = 1000 (D)
xy = 1000
5. Information: M is a concentration
unit equal to moles / volume (in Liters).
How many grams of solid sodium chloride
are needed to prepare 2.5 L of a 0.5M solution of sodium chloride?
The mass of 1 mole of sodium chloride is 58.5 g.
6. a. Using only the following
set of circles, show how you would represent the operation 6 ÷
2:
O O O O O O
b. Based on the definition
of division, draw a picture that justifies the answer to the following
problem. What is the answer to the following problem: 2 ÷
1/2?
7. a. Consider the ratio the
ratio $40 / 5 gallons. When we divide 40 by 5 we get 8.
What does 8 represent here?
b. Now consider
5 gallons / $40. 5/40 = 0.125. What does the 0.125 represent
in this case?
8. A certain liquid costs $5 per gallon.
What does the following equation represent: $40 / ($5 / 1 gallon)?
What is the answer to the above problem?
When you perform the division, what is the label on the number you obtain?
Unit 1: Introduction to Chemistry’s
Measurement and Problem Solving
Part 5 – Assignment: Let’s Start Problem Solving
Read pages 83 – 88 in your textbook. Then answer the following
questions (from your text).
1. Evaluate the following and record the answer.
a. 892
+ 173 + 56
=
b. (35
+ (Square root of 529)) / (2.9x1017)
=
c. The
volume of a sphere with radius r is given by (4/3)(pi)r3.
What is the volume of a sphere with a radius of 3.50 cm?
d. Find
the number of atoms in 7.00 g of gold if an atom of gold has a mass of
3.271x10-22 g.
2. The density of silicon is 2.33 g/cm3.
What is the volume of a piece of silicon that has a mass of 62.9 g?
3. A small piece of gold has a volume of 1.35
g/cm3.
a. What is the mass of the
gold piece, given that the density of gold is 19.3 g/cm3?
b. What is the value of the
gold piece if the market value of gold is $11/g?
4. A watch loses 0.15 seconds every minute.
How many minutes will the watch lose in one day?
5. Earth is approximately 1.5x108
km from the sun. How many minutes does it take light to travel from
the sun to Earth? The speed of light is 3.00x108
m/s.
Use the following procedure when solving mathematical problems
in this chemistry class.
1. Read the whole problem.
(Don’t just start writing down numbers.)
2. Write down what you are
given. (It is important that you known what you have to work with.)
3. Identify and write down
what you wish to find. (You can’t solve it if you don’t know what
you’re looking for.)
4. Is there an equation that
relates the given information to the unknown? Write the formula.
(More than one formula may be required. Write all required formula.)
5. Substitute the known values
into the equation. Make sure you are including units.
6. Write your final answer
using the correct number of significant figures and a correct unit.
Is it clearly recognizable as the answer or should you circle or box it?
7. Check to see that the answer
makes sense. If it doesn’t make sense, check over your work.
Unit
1: Introduction to Chemistry’s Measurement and Problem Solving
Part 6 – Notes: The International System of Units
Objectives: List the seven basic units
in the metric system, what they measure, and the abbreviations for their
units.
Compare units in the metric system
with units in the English system.
List the required metric prefixes,
their abbreviations, and their numerical values.
Distinguish between mass and weight.
Convert between temperatures in Celsius
and Kelvin scales.
Text Reference: Section 3.3 – pages 63-67
and Section 3.5 – pages 74-75
You are familiar with most common English units. Within
the English system, you need to know the following:
1 foot = 12 inches
1 minute = 60 minutes
1 yard = 3 feet
1 hour
= 60 minutes
1 mile = 5280 feet
1 day = 24 hours
1 year = 365 days
In the realm of science, the metric system is preferred for two
different reasons:
1. Metric units for complex
quantities are defined in terms of units for simpler quantities.
2. The metric system has the
same numerical base as the decimal system. Every unit in the metric
system is ten times the size of the next smaller unit. The units
in the metric system are based on the number 10.
There are seven basic quantities that may be measured. The
unit attached to a number indicates what quantity is being measured.
The seven basic quantities and their base units are as follows.
You need to know these.
Quantity
Modifiable Unit Unit Abbreviation
length
meter
m
mass
gram
g
time
second
s
temperature
kelvin
K
amount of substance
mole
mol
electric current
ampere
A
luminous intensity
candela
cd
In the metric system, every unit has a quantity that can be
modified by prefixes. There are specific prefixes used to modify
the units and these prefixes have specific prefixes. You need to
know the following prefixes and their values:
Prefix
|
Abbrev.
|
Meaning
(words)
|
Meaning
(math)
|
Or, for the
unit METER. . .
|
mega-
|
M
|
one million
|
1 000 000
|
1 Mm =
1 000 000 m
|
kilo-
|
k
|
one thousand
|
1 000
|
1 km =
1000 m
|
hecto-
|
h
|
one hundred
|
100
|
1 hm =
100 m
|
deka-
|
da
|
ten
|
10
|
1 dam =
10 m
|
deci-
|
d
|
one-tenth
|
0.1
|
1 m = 10
dm
|
centi-
|
c
|
one-hundredth
|
0.01
|
1m = 100
cm
|
milli-
|
m
|
one-thousandth
|
0.001
|
1 m = 1000
mm
|
micro-
|
µ
|
one-millionth
|
0.000 001
|
1 m = 1
000 000 µm
|
nano-
|
n
|
one-billionth
|
0.000 000
001
|
1 m = 1
000 000 000 nm
|
pico-
|
p
|
one-trillionth
|
0.000 000
000 001
|
1 m = 1
000 000 000 000 pm
|
Mass versus Weight:
Temperature:
Unit 1: Introduction to Chemistry’s
Measurement and Problem Solving
Part 6 – Assignment: The International System of Units
1. As you climbed a mountain and the force of
gravity decreased, would your weight increase, decrease or remain the
same? How would your mass change?
2. List these units in order from largest to
smallest: 1 dm3 1 mL
1 mL 1 L 1 cL 1
dL
__________ _________
__________ __________ __________
__________
largest
3. The boiling point of elemental argon is 87
K. What is argon’s boiling point in degrees Celsius?
4. Order these lengths from smallest to largest:
cm mm km
mm m nm dm
pm
________ ________ ________
________ ________ ________
________ ________
smallest
5. From what unit is a measure of volume derived?
6. Astronauts in space are said to have apparent
weightlessness. Explain why it is incorrect to say they are massless.
7. Which would melt first. germanium with a melting
point or 1210K or gold with a melting point of 1064oC?
8. Which is larger? Circle the larger of
the two measurements.
a. 1 centigram or 1 milligram
b.
1 liter or 1 centiliter
c. 1 calorie or 1 kilocalorie
d.
1 millisecond or 1 centisecond
e. 1 cubic millimeter or 1
cubic decimeter f.
1 microliter or 1 milliliter
Unit 1:
Introduction to Chemistry’s Measurement and Problem Solving
Part 7 – Notes: Unit Conversions
Objectives: Perform one-step and multi-step
units conversions with various units using dimensional analysis.
Text Reference: Section 4.2 – pages 89-95
and Section 4.3 – pages 97-100
Let’s look at converting between different units of length.
For all your work in chemistry, you will always perform conversions
using UNIT ANALYSIS. Unit analysis is a way you will solve
problems; it is also called dimensional analysis, unit factoring,, the
factor label method, and has a few other names.
When you solve problems using unit analysis, you begin with a known
quantity and use a ratio in a fraction form to allow you to convert from
your known unit to your unknown unit. When working with unit analysis,
it is essential that you keep track of and record all units.
To solve a problem, you need to know the ratio that exists between
your known and your unknown quantity. The ratio is an equality
with your known unit on one side of the equal sign and the unknown unit
of the other and the numbers in from of the units that make the equality
true. For example, if I wanted to convert between meters and centimeters,
I would need the following equality: 1 m = 100 cm. In some cases,
you may need to use multiple ratios to solve a problem. Let’s see
how to use this method of problem solving.
Example 1: Convert 3.654 days into
its equivalent quantity in hours.
Example 2: Convert 124.57 hours
into its equivalent quantity in days.
Example 3: Convert 376 cm into
its equivalent quantity in meters.
Example 4: Convert 475 mm into
its equivalent quantity in hectometers.
Example 5: Convert 568 meters into
its equivalent quantity in miles. Note 1 km = 0.62 mi.
Be careful when canceling units. Look at the following examples.
Example 6: Convert 0.748 m2
into its equivalent quantity in cm2.
Example 7: Convert 17564 mm3
into its equivalent quantity in m3.
Example 8: Convert 1.67 mi2
into its equivalent quantity in m2.
Remember: MEASUREMENT = QUANTITY + UNIT
While you pursue your work in chemistry, remember,
you are using measurements. This means you will be working with
values that have units attached. Keep your units with your numbers.
Units will have to cancel in order for you to obtain a valid answer.
In chemistry, NO NAKED NUMBERS!!!
Unit 1: Introduction to Chemistry’s
Measurement and Problem Solving
Part 7 – Assignment: Units Conversions
Solve the following using
unit analysis. Show all work, units, set-ups, etc. Convert:
1. 0.0004 kg to mg
2. 0.00000000006 Mm to km
3. 0.452 kL to cL
4. 256 dam to km
5. 3.5 hours to seconds
6. 3.76 years to minutes
7. 321 000 mm to pm
8. 12 385 884 seconds to years
9. 84 937 mL to nL
10. 0.38 km2
to mm2
11. 57 561 cm3
to Mm3
12. 0.00057 hm3
to dm3
Unit 1:
Introduction to Chemistry’s Measurement and Problem Solving
Part 8 – Notes: Density and Problem Solving
During your tour through chemistry, you will come upon numerous
problems that you will be required to solve. You need to solve the
mathematical problems in a logical fashion, with steps based on the scientific
method. Use the following procedure when solving mathematical problems
in this chemistry class.
1. Read the
whole problem. (Don’t just start writing down numbers.)
2. Write down
what you are given. (It is important that you known what you have
to work with.)
3. Identify
and write down what you wish to find. (You can’t solve it if you
don’t know what you’re looking for.)
4. Is there
an equation that relates the given information to the unknown?
Write the formula. (More than one formula may be required.
Write all required formula.)
5. Substitute
the known values into the equation. Make sure you are including
units.
6. Write your
final answer using the correct number of significant figures and a correct
unit. Is it clearly recognizable as the answer or should you circle
or box it?
7. Check to
see that the answer makes sense. If it doesn’t make sense, check
over your work.
DENSITY
What weighs more, a pound of feathers or a pound of steel?
You know the answer is “neither,” both weigh the same: 1 pound.
But if your instinct was to say “steel,” you were probably thinking of
the quantity of density. Steel is more dense than feathers.
What does that mean?
Density is defined as the mass per unit volume of a substance:
Density = mass / volume
D = m/V
The dimensions (combination of units)
of density involve a mass unit divided by a volume unit, such as g/mL or
g/cm3. So, to get the density
of an object, simply divide it mass by its volume.
NOTE: The density of pure
water is about 1.00 g/mL or 1.00 g/cm3.
Let’s now solve several density problems using the problem solving
steps from above. Remember, use all problem solving steps.
Example 1: Calculate the density,
in g/mL of wood in a desk if it has a mass of 30.0 kg and a volume of 35.0
L?
Example 2: Calculate the density
of a rectangular metal bar that is 5.00 cm long, 20.0 mm wide, and 0.0100
m thick and has a mass of 23.0 g. Calculate the density in g/cm3.
Example 3: Calculate the mass,
in kilograms, of 254 mL of mercury. The density of mercury is 13.6
g/mL.
Substances generally expand when heated, and the resulting change
in volume causes some change in density. Within reasonable temperature
changes, the density of a substance is fairly constant. For example
water varies from 0.99979 g/mL at 0oC
to 1.0000 g/mL at 4oC to 0.95838
g/mL at 100oC. You will
often ignore such changes.
Density is an intensive property, useful in identifying
substances. For example gold may be distinguished from iron pyrite
(fool’s gold) by their differing densities. The density of gold
is 19.3 g/mL while that of iron pyrite is only 5.0 g/mL.
The relative densities determine whether an object will float in
a given liquid in which it does not dissolve. An object will float
if its density is less than the density of the liquid in which it is
placed. An object will sink if its density is greater than the
density of the liquid in which it is placed. For example, the density
of water is 1.00 g/mL and a piece of wood placed in the water has a density
of 0.80 g/mL. The wood will float in the water since it has a lower
density.
Example 4: Will the wood in example
1 sink or float in water? Will it sink or float in mercury?
Specific Gravity:
Unit 1: Introduction to Chemistry’s
Measurement and Problem Solving
Part 8 – Assignment: Density and Problem Solving
Solve the following problems.
Show all work, set-ups, units, steps, etc. Use correct significant
figures. BE NEAT.
1. A cube of gold that is 2.000 cm on each side
has a mass of 154.4 g. What is the density of gold?
2. A piece of silver (density = 10.5 g/mL) dropped
in water and displaces 21.56 mL. What is the mass, in grams, of
the silver?
3. Concentrated sulfuric acid has a density of
1.84 g/mL. Calculate the mass, in grams, of 1.00 L of sulfuric
acid.
4. Calculate the density, in g/cm3,
of a block of wood that has a mass of 75.0 kg and the following dimensions:
length = 25.0 cm, width = 0.100 m, and depth = 550.0 mm.
5. A 1.0000 kg of metallic osmium, the “heaviest”
element known, occupies a volume of 44.5 cm3. Calculate the density
of osmium in g/cm3.
6. A student finds a piece of metal she thinks
is aluminum. In the lab, she determines the metal sample has a
mass of 612 g and a volume of 245 mL. The density of aluminum is
2.7 g/mL. Is the sample aluminum or not? Explain.
7. Would the density of a person be the same
on the surface of the moon as it is on earth? Explain.
8. Why doesn’t specific gravity have a unit?
9. If ice were more dense than water, it would
certainly be easier to pout water from a pitcher of ice cubes and water.
Can you imagine situations where this density switch would be a greater
problem?
10. The mass of a cube of iron is 355 g.
Iron has a density of 7.87 g/cm3.
What is the mass of a cube of lead that has the same dimensions as the
iron? Lead’s density is 11.4 g/cm3.
Unit 1:
Introduction to Chemistry’s Measurement and Problem Solving
Part 9 – Notes: Accuracy and Precision
Objectives: Define, explain, and distinguish
between accuracy and precision.
Calculate the absolute and relative
error and the absolute and relative deviation of a set of measurements.
Text Reference: Section 3.2 – pages 54-55
Two important factors to consider in measurement are precision
and accuracy. They are NOT the same thing.
Precision indicates the reproducibility of
a measurement.
Example:
Imagine you took three readings on a thermometer to try to determine
the boiling point of pure water at sea level. The results of your
trials were 96.8oC, 96.9oC,
and 97.2oC. Note how close
the measurements are to one another. The measurements show a high
degree of reproducibility, they are precise.
Accuracy indicates how close a measurement
is to its accepted value.
Consider the boiling point of pure water at
sea level in the example above. It is an accepted fact that pure
water at sea level boils at exactly 100oC. The three
measurements above, although precise, are not close to the accepted
value. They are not accurate.
Determining Accuracy – Calculating Percent
Error
Recall, accuracy deals with how close measurements are to
the accepted or actual value, so you will be comparing the measurements
to the accepted value.
Formula for Absolute Error:
(also called experimental error)
Formula for Percent Error:
(also called relative error)
From the example above we have three different values: 96.8oC,
96.9oC, and 97.2oC.
Calculate the percent error for each individual measurement:
Then calculate the
average of those measurements:
You can also calculate the percent error for the mean of the measurements:
What is the mean
of the measurements?
What is the percent
error for the mean of the measurements?
Determining Precision – Calculating
Deviation
Calculating how precise measurements are is a multi-step
process. You will again use the three temperature readings for the
boiling water example at the beginning of the notes to complete the steps
that follow. Recall, precision deals with how close measurements
are to one another, so you will be looking at comparing the measurements
to the average.
STEP 1: Determine
the Mean of the measurements.
STEP 2: Calculate the Absolute
Deviation for EACH measurement. In this step we are determining
how much each individual measurement deviates from the average.
STEP 3: Calculate the Average
Absolute Deviation.
STEP 4: Express the measurement
in terms of Mean ± Average Absolute Deviation.
STEP 5: Calculate the Relative
Deviation. (Also referred to as percent deviation.)
STEP 6: Express the measurement
in terms of Mean ± Relative Deviation.
Unit 1: Introduction to Chemistry’s
Measurement and Problem Solving
Part 9 – Assignment: Accuracy and Precision
On a separate sheet
of paper, answer the following questions. Be sure to show
all work, set-ups, steps, answers, etc. Be sure to keep strict attention
to significant figures.
1. In your own words, describe the difference
between accuracy and precision.
2. Why is the percent error of a measurement
always positive?
3. Comment on the accuracy and precision of the
these basketball free-throwers:
a. 99
of 100 shots are made.
b.
99 of 100 shots hit the front of the rim and bounce off.
c.
33 of 100 shots are made; the rest miss.
4. Three students made multiple massings of a
copper cylinder, each using a different balance. The correct mass
of the cylinder had been previously determined to be 47.32 g. Describe
the accuracy and precision of each student’s measurements.
Mass of Copper Cylinder
|
Lisa
|
Lamont
|
LeighAnn
|
Mass 1
|
47.13 g
|
47.45 g
|
47.95 g
|
Mass 2
|
47.94 g
|
47.39 g
|
47.91 g
|
Mass 3
|
46.83 g
|
47.42 g
|
47.89 g
|
Mass 3
|
47.47 g
|
47.41 g
|
47.93 g
|
5. Thinking there is a problem with one of the
minting machines, a US mint quality control inspector took the masses
of 10 new pennies. The recorded masses were:
3.112 g
3.129 g 3.093 g
3.089 g 3.109
g
3.089 g
3.094 g 3.085 g
3.131 g 3.092
g
a. Calculate
the mean.
b. Calculate
the average absolute deviation. (First find the absolute deviation
for each mass.)
c. Calculate
the relative deviation.
According to the US mint, the mass
of a new penny should be 3.160 g.
d. Calculate
the absolute error of the mean of the massing of the pennies.
Use the mean value.
e. Calculate
the percent error of the mean of the massing of the pennies. Use
the mean value.
6. Calculate the percent error for the measurements
in each of the following conditions.
a. The density
of an aluminum block was determined in an experiment as 2.64 g/mL.
The actual density of aluminum is 2.70 g/mL.
b. The experimental
determination of iron in an iron ore was 16.48%. The actual value
of iron present in the ore is 16.12%.
c. A balance
measures a 1.0000 g mass as 0.9981 g.