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

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