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Chapters 10 and 11 Problem Set Key: Percents and Introduction to
Concentration
1.
2.
3.
4.
5.
PART A: BASIC MANIPULATIONS WITH PERCENTS
Express the following percents as fractions AND as decimals:
5%
fraction = 5/100
decimal = 0.05
0.5%
fraction = 0.5/100
decimal = 0.005
0.05%
fraction = 0.05/100
decimal = 0.0005
0.341%
fraction = 0.341/100
decimal = 0.00341
2.433%
fraction = 2.433/100
decimal = 0.02433
Express the following fractions or decimals as percents:
6. 20/50
percent = 40%
7. 4/3
percent = 133.33%
8. 2.0/100
percent = 2%
9. 0.225
percent = 22.5%
State the following as percents:
10.
1 part in 100 total parts
percent = 1%
11.
2 parts in 25 total parts
percent = 8%
12.
18 parts in (18 parts + 42 parts)
percent = 30%
13.
0.05 parts in 1 part total
percent = 5%
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PART B: PERCENT ERROR
1. You are doing a new laboratory test for the first time and are trying to
determine your accuracy. The test examines the level of calcium in a
sample. You receive a sample that has been provided as a reference, and
is known to be at a concentration of 23.0 mg calcium/L. You test this
reference sample 10 times and get the following calcium concentrations:
23.0 mg/L
23.3 mg/L
23.0 mg/L
23.1 mg/L
22.8 mg/L
23.2 mg/L
23.1 mg/L
23.0 mg/L
22.9 mg/L
23.1 mg/L
Determine the percent error of your results, given the formula below:
% error = experimental value - true value X 100%
true value
average = 23.1 mg/L (not rounded = 23.05)
% error = 23.1 mg/L – 23.0 mg/L X 100% = 0.43%
23.0 mg/L
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PART C: Introduction to Concentration
1. Describe how to make 500 mL of 250 g/L NaCl.
0.5 L(250 g/L) = 125 g
Dissolve 125 g NaCl in enough water to make 500 mL total solution
2. Describe how to make 100 mL of a 100 g/mL glucose solution.
100 mL (100 g/mL) = 10,000 g
Dissolve 10,000 g (10 mg) glucose in enough water to make 100 mL total
solution
3. How much KOH is present in 5 L of a 0.1 g/mL solution?
5000 mL (0.1 g/mL) = 500 g KOH
4. How much enzyme is present in 150 L of a 250 g/mL enzyme
solution?
150 L ( 250 g/mL)(1 mL/1000 L) = 37.5 g enzyme
5. Which of the following concentrations is equal to 500 g/mL MgCl2?
a. 500 mg/L(1L/1000mL)(1000 ug/mg) = 500 ug/mL
b. 0.5 mg/mL (1000 ug/mg) = 500 ug/mL
c. 50 g/L (1 L/1000 mL) = 0.05 ug/mL
d. 5000 ng/mL (1 ug/1000 ng) = 5 ug/mL
e. 0.05 g/mL (106 ug/g) = 50,000 ug/mL