The percent composition gives the number of grams of an element present in the compound per 100g of the compound; the formula gives the relative number of moles of an element in one mole of the compound. **Percent Composition Formula** calculates the relative masses of each of the elements in a substance expressed as a percentage of total mass of the substance.

For example, the formula given below is used to calculate the percent composition for the element X.

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Solved examples based on the percent composition is given below. ### Solved Examples

**Question 1: **What is the percent composition of chromium in K_{2}Cr_{2}O_{7}?

** Solution: **

The formula weight of K_{2}Cr_{2}O_{7} is

2(39g/mol) + 2(52g/mol) + 7(16g/mol) = 294g/mol

% Composition of Cr = $\frac{2 \times 52}{294} \times$ 100%

% Composition of Cr = 35.4%

**Question 2: **Combustion of a 0.3037g sample of a certain compound yielded 1.0441g of CO_{2} and 0.1708g of H_{2}O. What is the percent composition of the compound?

** Solution: **

The carbon in CO_{2} is 12/44 of the mass of the CO_{2} and the hydrogen in H_{2}O is 2/18 of the mass of the H_{2}O.

Therefore mass of carbon in the CO_{2} = $\frac{12 \times 1.0441}{44}$ = 0.2848g

and mass of hydrogen in the H_{2}O = $\frac{2 \times 0.1708}{18}$ = 0.01898g

Thus % carbon in the sample = $\frac{0.2848 \times 100}{0.3037}$ = 93.7%

and % hydrogen in the sample = $\frac{0.01898 \times 100}{0.3037}$ = 6.25%

The formula weight of K

2(39g/mol) + 2(52g/mol) + 7(16g/mol) = 294g/mol

% Composition of Cr = $\frac{2 \times 52}{294} \times$ 100%

% Composition of Cr = 35.4%

The carbon in CO

Therefore mass of carbon in the CO

and mass of hydrogen in the H

Thus % carbon in the sample = $\frac{0.2848 \times 100}{0.3037}$ = 93.7%

and % hydrogen in the sample = $\frac{0.01898 \times 100}{0.3037}$ = 6.25%