Self-Assessment and Practice
Check your solutions by clicking on the word Answer.
1. Calculate the percent composition of HCl.
When you are only given a compound formula, start by calculating the molar mass of the compound.
1.0 g + 35.5 g = 36.5 g (molar masses from the periodic table)
H (1.0 g/36.5 g) x100% = 2.7% H
Cl (35.5 g/36.5 g) x 100% = 97.3% Cl
2. Calculate the percent composition of K2CO3
2(39.1 g) + 12.0 g +3(16.0 g) = 138.2 g
%K = (78.2g/138.2 g) x 100% = 56.6% K
%C = (12.0 g/138.2 g) x 100% = 8.68% C
%O = (48.0 g/138.2 g) x 100% = 34.7% O
3. Calculate the percent composition for Mg(OH)2.
24.3 g + 2(16.0 g) + 2(1.0 g) = 58.3 g
%Mg = (24.3g/58.3g) x 100% = 42.7%
%O = (32.0g/58.3g) x 100% = 54.9%
%H = (2.0g/58.3g) x 100% = 3.4%
4. Calculate the percent composition for potassium sulfate.
You must first be certain that you have the correct formula for potassium sulfate. Refer to an ionic charges chart.
2(39.1 g) + 32.1 g + 4(16.0 g) = 174.3 g
%K = (78.2 g/174.3 g) x 100% = 44.9%
%S = (32.1 g/174.3 g) x 100% = 18.4%
%O = (64.0 g/174.3 g) x 100% = 36.7%
Work each of the following problems.
1. A compound is found to contain 63.52% iron and 36.48% sulfur. Find its empirical formula.
When you are given the percentage composition of a compound and asked to find the empirical formula, just assume that you have 100g of the substance. This allows you to turn your % into grams. Out of 100 g of this compound of iron and sulfur, 63.52g are iron and 36.48g are sulfur.
2. In the laboratory, a sample is found to contain 1.05 grams of nickel and 0.29 grams oxygen. Determine the empirical formula.
3. What is the empirical formula for a compound if a 2.50 g sample contains 0.900 g of calcium and 1.60 g of chlorine?
4. A compound is 85.7 % carbon and 14.3 % hydrogen. Calculate the empirical formula.