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
