When active metals such as magnesium are immersed in acid solution, hydrogen gas is evolved. Calculate the volume of H₂(g) at 30.1°C and 0.85 atm that can be formed when 275 mL of 0.725 M HCl solution reacts with excess Mg to give hydrogen gas and aqueous magnesium chloride. Mg(s) + 2HCl(aq) → MgCl₂(aq) + H₂(g)
(R = 0.08206 L • atm/K • mol)

Question

Quizplus · Accepted Answer

First, we need to determine the number of moles of HCl solution used:

275 mL (1 L/1000 mL) = 0.275 L HCl solution
0.725 mol/L HCl solution × 0.275 L = 0.200 moles of HCl solution

Since excess Mg is used, the limiting reactant is HCl solution, so we can calculate the number of moles of H2 gas formed using the stoichiometry of the balanced equation:

0.200 mol HCl solution × (1 mol H2 gas/2 mol HCl solution) = 0.100 mol H2 gas

Then we can use the ideal gas law to calculate the volume of H2 gas at the given conditions:

PV = nRT

V = nRT/P = (0.100 mol)(0.08206 L•atm/K•mol)(303.1 K)/(0.85 atm) = 2.88 L

Rounding to the nearest tenth gives a final answer of 2.9 L, which is choice C.

When Active Metals Such as Magnesium Are Immersed in Acid