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A sample of hydrogen peroxide is labeled as '5.6 volume'. Its density is 1 g/mL. Express its concentration as (i) mass percentage and (ii) molarity. (Molar mass of H2O2 = 34 g/mol)

1. 1.7% and 0.25 M
2. 1.7% and 0.5 M
3. 0.85% and 0.5 M
4. 0.85% and 0.25 M

Correct answer: 1.7% and 0.5 M

Solution

From 2H2O2 -> 2H2O + O2, 1 mol O2 requires 2 mol H2O2. 5.6 mL O2 at STP = 5.6/22400 mol O2 per mL solution, giving moles H2O2 per mL = 2 * 5.6/22400 = 0.0005 mol. Molarity = 0.0005 * 1000 = 0.5 M. Mass of H2O2 per mL = 0.0005 * 34 = 0.017 g. Mass% = (0.017/1)*100 = 1.7% (since density = 1 g/mL, 1 mL = 1 g).

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