Question

a) Magnesium metal reacts with HCl solution, liberating H₂ gas and generating Mg²⁺ cations in solution. A 2.780-g sample of Mg metal is added to 50.0 mL of a 4.00 M HCl solution, and the reaction goes to completion. What mass of H₂ is formed?

b) What is the concentration of Mg²⁺ ions in the resulting solution? (Assume the final volume of solution is still the same as the original volume of the acid.)

Answer 1

(a)

Mg mass = 2.780 g

Mg molar mass = 24 g/mol

Mg moles = mass / molar mass = 2.780 / 24

                 = 0.116

HCl moles = 4 x 50 / 1000 = 0.2

Mg + 2 HCl -------------------> MgCl2 + H2

1 mol Mg = 2 mol HCl

0.116 mol Mg = 0.2 mol HCl are present

here limiting reagent HCl

so for

2 mol HCl -------------------------> 1 mol H2

0.2 mol HCl -----------------------> x mol H2

x = 0.2 x 1/2 = 0.1 mol H2

H2 moles = mass / molar mass

0.1 = mass / 2

mass = 0.2 g

H2 mass = 0.2 g

b)

2 mol HCl --------------------> 1 mol Mg+2 formed (MgCl2)

0.1 mol HCl ---------------------> y mole Mg+2

y = 0.2 /2 = 0.1

moles of Mg+2 = 0.1

concentration = moles / volume (Litre)

                       = 0.1 / 50 x 10^-3

                        = 2 M

Mg+2 concentration = 2 M