Question

What mass of magnesium chloride is required to make 1000. mL of a solution that is 2.50 ×102M in chloride ions? Assume the magnesium chloride completely dissociates.

Answer 1

Magnesium chloride is MgCl₂

One molecule of MgCl₂ will two 2 Cl^- ions on dissociation

MgCl2 Mg²⁺ + 2 Cl^-

To make a solution of 2.5x10^-2 M chloride ion, we require (2.5x10^-2 /2) M, MgCl₂ solution.

2.5x10^-2 /2 = 1.25 x 10-2

Molarity = No. of moles / Volume of solution (in L)

(No. of moles = m/M)

Molarity = m / M x V

(Volume given 1000 ml = 1L and Molecular mass of MgCl2 = 95.2)

1.25 x 10^-2 = m / 95.2 x 1

m = 1.25 x 10^-2 x 95.2 = 1.19 g

Mass of magnesium chloride is required is 1.19 g