Question

A solution was prepared by dissolving 2.2 g of an unknown solute in 16.7 g of CCl4. A thermal analysis was performed for this solution and it was found that its initial freezing point was – 28.7°C. A reliable source in the bibliography states that for CCl4, T°f = – 22.9°C, and its freezing point lowering constant is Kf = 29.9°C/m. Calculate the molar mass of the unknown solute.

Answer 1

Answer:

Molar mass of unknown solute is 679 g/mol

Explanation:

Let us assume that the solute is a non-electrolyte.

For a solution with non-electrolyte solute remains dissolved in it -

Depression in freezing point of solution, $\Delta T_{f}=K_{f}.m$

where, m is molality of solute in solution and $K_{f}$ is cryogenoscopic constant of solvent.

Here $\Delta T_{f}=(-22.9^{0}\textrm{C})-(-28.7^{0}\textrm{C})=5.8^{0}\textrm{C}$

If molar mass of unknown solute is M g/mol then-

                 $m=\frac{\frac{2.2}{M}}{0.0167}mol/kg$

So, $5.8^{0}\textrm{C}=29.9^{0}\textrm{C}/(mol/kg)\times \frac{\frac{2.2g}{M}}{0.0167}mol/kg$

so, M = 679 g/mol