A solution contains H2O2 in water. Complete decomposition of the H2O2 in 170 g of this solution yields 5.6 L of O2(g), measured at 0 oC and 101.325 kPa. What is the percentage by mass of H2O2 in this solution? Choose the closest answer.

Question

A 10% B 20% C 5.0% D 3.0% E 1.0%

Answer 1

2H2O2 ==> 2H2O + O2

mols O2 = 5.6 x (1 mol/22.4L) = about 0.26 but that's an estimate and you should work the math and get a better answer.
Convert mols O2 to mols H2O2. That's mols O2 x 2 = mols H2O2.
grams H2O2 = mols H2O2 x molar mass H2O2 = ?
Then %H2O2 = (grams H2O2/mass sample)*100 = ?