Final answer:
To produce two moles of hydrogen gas in the reaction of magnesium with hydrochloric acid, four moles of HCl are required as per the stoichiometry of the balanced chemical equation.
Step-by-step explanation:
The reaction of solid magnesium (Mg) with hydrochloric acid (HCl) produces magnesium chloride (MgCl₂) and hydrogen gas (H₂). According to the balanced chemical equation Mg(s) + 2 HCl(aq) → MgCl₂ (aq) + H₂(g), to produce one mole of hydrogen gas, two moles of hydrochloric acid are needed. Therefore, to produce two moles of hydrogen gas, you would need twice the amount of hydrochloric acid, which equals 4 moles of HCl. This is because the stoichiometry of the reaction dictates that for every mole of hydrogen gas produced, two moles of HCl are consumed in the reaction.