A 5.00 g sample of magnesium reacts with excess hydrochloric acid to produce hydrogen gas and magnesium chloride. How many moles of H2 are produced? (Mg molar mass = 24.305 g/mol)

• A. 0.103 mol
• B. 0.412 mol
• C. 0.206 mol
• D. 0.250 mol

Answer: (C)

In this reaction, magnesium reacts with excess hydrochloric acid to produce hydrogen gas, and the balanced equation shows a 1:1 relationship between magnesium and hydrogen gas. That means the number of moles of hydrogen produced equals the number of moles of magnesium that react. First find how many moles of magnesium you have: 5.00 g ÷ 24.305 g/mol ≈ 0.206 mol. Since the acid is in excess, magnesium is the limiting reagent, so all of it is converted to hydrogen gas. Therefore, about 0.206 moles of hydrogen gas are produced.