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A B

C D
FIGURE 10.3 If you know the name of an ingredient, you can write a chemical formula, and the percent composition of a particular
substance can be calculated from the formula. This can be useful information for consumer decisions.

The mass percentage of calcium in CaCO 3 can be found in
two steps: SOLUTION
Step 1: Write the formula for sodium fluoride, NaF.
Step 1: Determine formula weight:
Step 2: Determine the formula weight.
Atoms Atomic Weight Totals
1 of Ca 1 × 40.1 u = 40.1 u Atoms Atomic Weight Totals
1 of Na 1 × 23.0 u = 23.0 u
1 of C 1 × 12.0 u = 12.0 u
1 of F 1 × 19.0 u = 19.0 u
3 of O 3 × 16.0 u = 48.0 u
Formula weight = 42.0 u
Formula weight = 100.1 u
Step 3: Determine the percentage of Na and F.
Step 2: Determine percentage of Ca:
For Na:
(40.1 u Ca)(1)
__ × 100% CaCO 3 = 40.1% Ca (23.0 u Na)(1)

__

100.1 u CaCO 3 42.0 u NaF × 100% NaF = 54.7% Na
Knowing the percentage of the total mass contributed by For F:
the calcium, you can multiply this fractional part (as a decimal) (19.0 u F)(1)
__

× 100% NaF = 45.2% F

by the mass of the supplement pill to find the calcium supplied. 42.0 u NaF
The mass percentage of the other elements can also be deter-
The percentage often does not total to exactly 100 percent because
mined with equation 10.1.
of rounding.
EXAMPLE 10.3 EXAMPLE 10.4
Sodium fluoride is added to water supplies and to some toothpastes Calculate the percentage composition of carbon in table sugar, sucrose,
for fluoridation. What is the percentage composition of the elements which has a formula of C 12 H 22 O 11 . (Answer: 42.1% C)
in sodium fluoride?
254 CHAPTER 10 Chemical Reactions 10-4