If 6.42 g of water is produced, how many grams of oxygen gas reacted?

5.76g

Explanation:

2H2 + O2 = > 2H2O

Molar mass of H2O = 18g/Mol

No of mole of H2O = 6.42/18 = 0.36 moles of water.

1 mole of O2 gives 2 moles of water.

Xmole of O2 gives 0.36 moles of water.

X = 0.36/2 = 0.18 mole

Molar mass of O2 = 32g/mol

Mass = 32 * 0.18 = 5.76g.