# Percent Composition Law of Definite Proportions The Law

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Percent Composition

Law of Definite Proportions � The Law of Definite Proportions states that a compound is always comprised of the same atoms in the same ratio, or percentage. ◦ In order to determine the percentages, we use molar mass

Percent Composition � Percent composition gives you the ratio of the mass of one element to all the other elements that make up a unique compound. ◦ Methane, CH 4, is comprised of 1 atom of carbon and 4 atoms of hydrogen. ◦ The molar mass of methane is 16. 04 g/mol ◦ Carbon contributes a total mass of 12. 01 g/mol ◦ All Hydrogen atoms together contribute 4. 03 g/mol ◦ The ratio of C: CH 4 is 12. 01: 16. 04 or 74. 88% ◦ The ratio of H: CH 4 is 4. 03: 16. 04 or 25. 12% � So therefore, the percent composition of methane is 74. 88% Carbon and 25. 12% Hydrogen

Practice problems 1. 2. 3. 4. 5. Calculate the % composition of Fe 3 C, a compound used to cast iron. Calculate the % composition of sulfur dioxide. Calculate the % composition of ammonium nitrate. Calculate the percent composition of glucose, C 6 H 12 O 6. Calculate the percent composition of acetic acid, CH 3 COOH.

Practice problems 1. ◦ ◦ ◦ Calculate the % composition of Fe 3 C, a compound used to cast iron. Fe = 55. 85 g/mol x 3 = 167. 6 g/mol C = 12. 01 g/mol Fe 3 C = 167. 6 g/mol + 12. 01 g/mol = 179. 6 g/mol % Fe = 167. 6 g/mol / 179. 6 g/mol x 100 = 93. 32% % C = 12. 01 g/mol / 179. 6 g/mol x 100 = 6. 69%

Practice Problem Answers 1. 2. 3. 4. 5. ◦ 93. 32% Fe, 6. 69% C 50. 05% S, 49. 95% O 35. 00% N, 5. 05% H, 59. 96% O (p. s. the formula is NH 4 NO 3) 39. 99% C, 6. 73% H, 53. 28% O !!!! What is the empirical formula for each? !!!

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