Chemistry Practice Problems Freezing Point Depression Practice Problems Solution: Sulfuric acid in water dissociates completely into...

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Solution: Sulfuric acid in water dissociates completely into H+ and HSO4- ions. The HSO4- ion dissociates to a limited extent into H+ and SO42-. The freezing point of a 0.1000 m solution of sulfuric acid in water is 272.72 K.Calculate the molality of SO42- in the solution, assuming ideal solution behavior.

Problem

Sulfuric acid in water dissociates completely into H+ and HSO4- ions. The HSO4- ion dissociates to a limited extent into H+ and SO42-. The freezing point of a 0.1000 m solution of sulfuric acid in water is 272.72 K.

Calculate the molality of SO42- in the solution, assuming ideal solution behavior.

Solution

We’re being asked to find the molality of SO42- solution assuming ideal behavior.

When calculating the freezing point of a solution, we’re going to use the equation for Freezing Point Depression.

Tf=i·Kf·m

∆Tf = change in freezing point = Tf pure solvent –Tf solution
Kf = freezing point depression constant
i = van' t Hoff factor of the solute = no. of ions
m = molality

Recall that the formula for molality is:

Molality, m=moles solute (mol)kilogram solvent (kg)

Determine the required values for the Freezing Point Depression equation:

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