Using this equation 2H2+O2->2H2O when 47g of water are produced, how many grams of hydrogen must react

You must react 5.3 g H_2 to produce 47 g H_2O.

Step 1. Calculate the moles of H_2O

Moles of H_2O = 47 g H_2O * (1 mol H_2O/18.02 g H_2O) = 2.61 mol H_2O

Step 2. Calculate the moles of H_2

Moles of H_2 = 2.61 mol H_2O * (2mol H_2/2 mol H_2O) = 2.61 mol H_2

Step 3. Calculate the mass of H_2

Mass of H_2 = 2.61 mol H_2 * (2.016 g H_2/1 mol H_2) = 5.3 g H_2