Question

Consider the reaction.

mc013-1.jpg

Which statement is true at STP? (The atomic mass of Mg is 24.31 u.)
3.0 g of Mg produce 3.0 L of H2 gas.
3.0 g of Mg produce 22.4 L of H2 gas.
24.31 g of Mg produce (3 mc013-2.jpg 22.4) L of H2 gas.
(3 mc013-3.jpg 24.31) g of Mg produce (3 mc013-4.jpg 22.4) L of H2 gas.

Answer 1

The reaction is Mg + 2HCl → MgCl2 + H2, so 1 mol Mg produces 1 mol H2. 1 mol Mg = 24.31 g and 1 mol gas at STP = 22.4 L. Therefore 24.31 g Mg → 22.4 L H2, so the third statement is true.

(For reference, 3.00 g Mg = 3.00/24.31 = 0.1234 mol → 0.1234·22.4 = 2.77 L H2, not 3.00 L.)