STC_Percent Composition Lesson

Percent Composition

To describe the relative masses of elements in a compound, we use percent composition. This is simply this mass ratio of each element to the mass of the whole compound or molecule. So, to calculate the % composition, you calculate the % by mass of each element.  

Formula % by Mass of an Element = Mass of Element Mass of Sample X 100

% Composition Example: A 4.10 g sample of a compound that contains chlorine and tin was found to contain 2.22 g of chlorine. What is the percentage composition of the compound? Solution: Mass of chlorine: 2.22 g Total mass of compound: 4.10 g Mass of tin: 4.10-2.22 g = 1.88 g % for Cl = 2.22 g 4.10 g x 100% 54.1% Cl 1.88 g % for Sn=- x 100% = 45.9% Sn 4.10 g You can check your work to be sure that the percentages add to 100%. You may be off by a small amount, but this is to be expected, due to rounding during calculations. Do remember to always round to the correct number of significant figures, but not until the end of a problem.

You can also calculate percent composition from a known formula. When the percent composition is determined from the chemical formula, it is called the theoretical percentage composition. An actual percentage composition might be calculated from lab data as was done above.

Theoretical % Composition Example: Calculate the percentage composition by mass of (NH4)2(S): Solution: Find the mass of the entire compound. Then divide the mass of each element by the mass of the whole sample. N = 2 x 14.01 u = 28.02 u H = 8 x 1.01 u = 8.08 u S = 1 x 32.07 u = 32.07 u Total = 68.17 u 28.02 น % for N = x 100% 41.10% N 68.17 u 8.08 u % for H = x 100% 11.85% H 68.17 u % for S = 32.07 u 68.17 u x 100% = 47.04% S

Empirical and Molecular Formulas

Given the experimental masses of a compound (as above), you can also determine what is known as the empirical formula or a compound or molecule. Empirical formula is the simplest ratio of moles (or atoms) of each element in that compound.

Take a look to see how empirical formulas are calculated.

Determining Empirical Formulas from Indirect Analysis

Indirect analysis of a compound is the most frequently encountered laboratory data. A combustion reaction is a common example of this. Using the steps you learned in the presentation above. See if you can do the following guided problem.

Remember to work on the module practice problems as you complete each section of content.

[CC BY-NC-SA 4.0 Links to an external site.] UNLESS OTHERWISE NOTED | IMAGES: LICENSED AND USED ACCORDING TO TERMS OF SUBSCRIPTION - INTENDED ONLY FOR USE WITHIN LESSON.