Question

A 19.04 g mixture of sugar (C12H22O11) and table salt (NaCl) is dissolved in 233 g of water. The freezing point of the solution was measured as -3.58 °C. Calculate the mass percent of sugar in the mixture. A list of Kf values can be found here.

Answer 1

Let x g of sugar are present in the the mixture. Mass of mixture is 19.04 g. Mass of NaCl in the mixture .

The molar masses of sugar and NaCl are 342.3 g/mol and 58.4 g/mol.

Number of moles of sugar

Number of moles of NaCl

Since 1 mole of dissociates to give two moles of ions in solution, we multiply number of moles of NaCl with 2 to obtain number of moles of ions from NaCl

Total number of moles of particles from sugar and NaCl

Mass of water is 233 g or

Molality of solution

.....(1)

The depression in the freezing point

Molality ......(2)

But (1)=(2)

Mass percent of sugar in the mixture