Determine the percent composition by mass of H2O.

Determine the percent composition by mass of H2O.

• A. H ≈ 11.19%; O ≈ 88.81%
• B. H ≈ 88.81%; O ≈ 11.19%
• C. H ≈ 50%; O ≈ 50%
• D. H ≈ 5%; O ≈ 95%

Answer: (A)

The percent composition by mass is found by comparing the mass of each element in one mole of the compound to the compound’s molar mass. For water, there are 2 hydrogens and 1 oxygen per molecule. Using H = 1.008 g/mol and O = 16.00 g/mol, the molar mass of H2O is 2(1.008) + 16.00 = 18.016 g/mol. The hydrogen mass is 2.016 g, so H% = (2.016 / 18.016) × 100 ≈ 11.19%. The oxygen mass is 16.00 g, so O% = (16.00 / 18.016) × 100 ≈ 88.81%. The percentages sum to 100%, and the larger oxygen mass reflects its greater atomic weight. Therefore, the distribution is about 11.19% hydrogen and 88.81% oxygen.

The percent composition by mass is found by comparing the mass of each element in one mole of the compound to the compound’s molar mass. For water, there are 2 hydrogens and 1 oxygen per molecule. Using H = 1.008 g/mol and O = 16.00 g/mol, the molar mass of H2O is 2(1.008) + 16.00 = 18.016 g/mol. The hydrogen mass is 2.016 g, so H% = (2.016 / 18.016) × 100 ≈ 11.19%. The oxygen mass is 16.00 g, so O% = (16.00 / 18.016) × 100 ≈ 88.81%. The percentages sum to 100%, and the larger oxygen mass reflects its greater atomic weight. Therefore, the distribution is about 11.19% hydrogen and 88.81% oxygen.