Question

Zinc and magnesium metal each reacts with hydrochloric acid to make chloride salts of the respective metals, and hydrogen gas. A 10.00-g mixture of zinc and magnesium produces 0.5492 g of hydrogen gas upon being mixed with an excess of hydrochloric acid. Determine the percent magnesium by mass in the original mixture.

Answer 1

let x= mass of Zinc (Zn), 10-x = mass of magnesium (Mg)

atomic weights : Zn = 65.4 and that of Mg= 24.3

moles = mass/molar mass

moles : Zn = x/65.4 =0.0153x , moles of Mg= (10-x)/24.3= 0.0411*(10-x)= 0.411-0.0411x

The reaction of Zn as well as Mg with excess HCl is

Zn + 2HCl -------->ZnCl2+ H2 and Mg + 2HCl --------> MgCl2+ H2

1 mole of Zn gives 1 mole of H2 with excess of HCl and similarly, 1 mole of Mg gives 1 mole of H2 with excess of H2

0.0153x moles of Zn gives 0.0153x mole of H2 and   0.411-0.0411x moles of Mg gives 0.411-0.0411x moles of H2

moles of H2 totally generated= 0.0153x+0.411-0.0411x=0.411-0.0258x

mass of hydrogen generated =moles* molar mass of Hydrogen =2*(0.411-0.0258)= 0.5492

0.822- 0.0516x= 0.5492

0.0516x= 0.822-0.5492

x= 5.3 gm= mass of Zn, mass of Mg= 10-5.3= 4.7