Question

In the reaction Mg (s) + 2HCl (aq) H2 (g) + MgCl2 (aq), how many liters of hydrogen gas will be produced from 75.0 milliliters of a 3.0 M HCl in an excess of Mg at STP?
a. 1.1 liters
b. 34 liters
c. 2.52 liters
d. 0.11 liters E. 0.23 liters

Answer 1

The answer is c. 2.52 liters.

Molarity is a measure of the concentration of solute in a solution.
It can be expressed as moles of solute ÷ volume of solution:
c = n ÷V
where:
c - concentration of solute,
n - moles of solute
V - volume of solution

We have:
c = 3 M
V = 75 ml = 0.075 l
n = ?
Any gas at STP occupies 22.4 l/mol.

If c = n ÷V, then
n = c × V
n = 3 M × 0.075 l
n = 0.225 moles HCl

In the reaction, there are 2 moles of HCl, so moles of H₂ would be the half of HCl moles:
n₁ = 0.255 moles HCl ÷ 2
n₁ = 0.1275 moles H₂

Thus, if n₁ = 0.1275 moles H₂, and Mg at STP occupies 22.4 l/mol, then
0.1275 moles H₂ × 22.4 l/mol = 2.52 l