Question

If 35 mL of a 1.20 M solution of hydrogen peroxide completely decomposes, how many grams of oxygen are produced?

Answer 1

2H2O2 --> 2H2O + O2

How many moles H2O2 do you have? That's moles = M x L = 1.20 M x 0.035 L = 0.042
The coefficients tell you that 2 mols H2O2 will produce 1 mols O2; threfore, 0.042 moles H2O2 will produce 1/2 * 0.042 = 0.021 mole O2.
grams O2 = mols O2 x molar mass O2 = ?