Question

Calculate the molality of 2.0 M MgCl2 solution. The density of the solution is 1.127 g/mL. (The molar mass of MgCl2 = 95.211 g/mol and the molar mass of H2O is 18.02 g/mol.)

Answer 1

The answer is 2.135 mol/Kg

Given that molarity is 2M, that is, 2 moles in 1 liter of solution.

Density of solution is 1.127 g/ml

Volume of solution is 1L or 1000 ml

mass of solution (m) = density × volume

m₁ = density × volume = 1.127 × 1000 = 1127 g

mass of solute, m₂ = number of moles × molar mass

m₂ = 2 × 95.211

m₂ = 190.422 g

mass of solvent = m₁ - m₂

= 1127 - 190.422

= 936.578 g

= 0.9366 Kg

molality = number of moles of solute / mass of solvent (in kg)

= 2 / 0.9366

= 2.135 mol/Kg

Answer 2

The answer is 2.135 mol/Kg