# An aqueous solution of Na2SO4 is prepared and its measured freezing point is -16.0o C. Calculate the molarity of the solution. The density of the solution is 1.10g/mL. Kf for water is 1.86o C/m.

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An aqueous solution of Na2SOis prepared and its measured freezing point is -16.0o C. Calculate the molarity of the solution. The density of the solution is 1.10g/mL. Kfor water is 1.86o C/m.

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Step 1

The depression or lowering in freezing point is expressed as:

The molality  and molarity are related as follows :

Step 2

Given,

Freezing point of solution, Tf = -16.0 oC

Freezing point of solvent (water), Tfo = 0 oC

Freezing point depression constant, Kf = 1.86 oC/m

Density of the solution = 1.10g/mL

Na2SO4 (aq) →2  Na+ (aq) + SO42- (aq)

Here, i = number of ions dissociated = 2 + 1 = 3

And,

△Tf = Freezing point of solvent – Freezing point of solution

△Tf = 0 oC – (–16.0 oC)

△Tf = 16.0 oC

Substituting the values of △Tf , i and Kf in equation (1), we get,

Step 3

Now, given density, d =  1.10g/mL

Molar mass of Na2SO4 , MB = (2×23) + 32 + (4×16) = 142 g/mol

The molarity can be calculated using equation (2) as follows :

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