Percent Composition by Mass

The percent composition by mass is the ratio of the masses of one element in a compound to the mass of the entire compound expressed as a percent. Due to the law of constant composition, the relative amount of an element in a compound is independent of the source of the compound. That means that we can use either experimentally measured masses or atomic weights from the periodic table; either source of data would five the same result.

From experimental results—to find the percent composition by mass from experimental data, divide the mass of the element by the mass of the compound and express the result as a percent.

Concept Check: 0.5627 grams of elemental silver are recovered from 0.8861 grams of an unknown compound. What is the percent silver by mass of this compound?

Answer: divide the mass of the elemental silver by the mass of the unknown compound to find the percent silver:

This answer has four significant figures by the division rule and the 100 is exact.

From a formula—to find the percent composition by mass from atomic weights divide the atomic weight of the total amount of the element in the compound by the formula weight (or molecular weight) of the compound and express the result as a percent.

Concept Check: What is the percent silver in silver carbonate? Is the unknown compound from the previous problem silver carbonate?

Answer: The formula of silver carbonate is Ag₂CO₃. Assume one mole.

%Ag = 100{2(107.8682 g)/[2(107.8682 g) + 12.0107 g + 3(15.9994 g)]} = 78.2376%

This is not the same compound because 63.50 ≠ 78.2376. The answer has seven significant figures. The small whole numbers and the 100 are exact numbers. The numerator has seven significant figures. The denominator has seven significant figures. Note that the number 12.0107 does not limit the significant figures because, by the addition rule, the result of the addition in the denominator is rounded to the fourth decimal place.

This site was last updated 08/04/06