Question

calculate the molarity of the chloride ion in a 0.960 m solution of Magnesium chloride that has density of 1.87g/ml

Answer 1

Molality of Magnesium Chloride solution = 0.960 m

Density of the solution = 1.87 g/mL

Molality = Number of moles of solute/mass of solvent (in kg)

Let's suppose 0.960 m means 0.960 moles of solute (MgCl₂) in 1000 g (1 kg) of solvent which is water in this case.

Molecular mass of MgCl₂ = 95 g/mol

Mass of MgCl₂ = mole of MgCl₂ x Molecular mass of MgCl₂

= (0.960 mol)x(95 g/mol) = 91.2 g

Mass of solution = Mass of solute + Mass of solvent

Mass of solution = 1000 g + 91.2 g = 1091.2 g

We know, density = mass / volume

Volume of solution = (1091.2 g)/(1.87 g/mL) = 583.53 mL

Molarity of MgCl₂ solution = (moles of MgCl2/Volume of solution (mL))x1000

Molarity of MgCl₂ solution = (0.906/583.53)x1000 = 1.55 M

Now, we have to calculate the molarity of Cl^- ion in the solution.

MgCl₂ (aq) ---> Mg²⁺ + 2 Cl^-

1 M 1 M 2M

Hence, molarity of Cl^- ion in the solution = 2x1.55 M = 3.10 M