EMPIRICAL, MOLECULAR FORMULAS AND PERCENTAGE COMPOSITION BY MASS ASSIGNMENT

1. What information do we need to determine the molecular formula of a compound from the empirical formula?
2. Calculate the following to four significant figures:

(a) the percent composition of ammonia, NH₃

(b) the percent composition of photographic “hypo,” Na₂S₂O₃

(c) the percent of calcium ion in Ca₃(PO₄)₂

3. Determine the following to four significant figures:

(a) the percent composition of hydrazoic acid, HN₃

(b) the percent composition of TNT, C₆H₂(CH₃)(NO₂)₃

(c) the percent of SO₄^2– in Al₂(SO₄)₃

4. Determine the percent ammonia, NH₃, in Co(NH₃)₆Cl₃, to three significant figures.
5. Determine the percent water in CuSO₄∙5H₂O to three significant figures.
6. Determine the empirical formulas for compounds with the following percent compositions:

(a) 15.8% carbon and 84.2% sulfur

(b) 40.0% carbon, 6.7% hydrogen, and 53.3% oxygen

7. Determine the empirical formulas for compounds with the following percent compositions:

(a) 43.6% phosphorus and 56.4% oxygen

(b) 28.7% K, 1.5% H, 22.8% P, and 47.0% O

8. A compound of carbon and hydrogen contains 92.3% C and has a molar mass of 78.1 g/mol. What is its molecular formula?
9. Dichloroethane, a compound that is often used for dry cleaning, contains carbon, hydrogen, and chlorine. It has a molar mass of 99 g/mol. Analysis of a sample shows that it contains 24.3% carbon and 4.1% hydrogen. What is its molecular formula?
10. Determine the empirical and molecular formula for chrysotile asbestos. Chrysotile has the following percent composition: 28.03% Mg, 21.60% Si, 1.16% H, and 49.21% O. The molar mass for chrysotile is 520.8 g/mol.
11. Polymers are large molecules composed of simple units repeated many times. Thus, they often have relatively simple empirical formulas. Calculate the empirical formulas of the following polymers:

(a) Lucite (Plexiglas); 59.9% C, 8.06% H, 32.0% O

(b) Saran; 24.8% C, 2.0% H, 73.1% Cl

(c) polyethylene; 86% C, 14% H

(d) polystyrene; 92.3% C, 7.7% H

(e) Orlon; 67.9% C, 5.70% H, 26.4% N

12. A major textile dye manufacturer developed a new yellow dye. The dye has a percent composition of 75.95% C, 17.72% N, and 6.33% H by mass with a molar mass of about 240 g/mol. Determine the molecular formula of the dye.