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

Molar mass of unknown solute is 679 g/mol

Step-by-step 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=((2.2)/(M))/(0.0167)mol/kg`

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

so, M = 679 g/mol