Question

1. The Rast method for determining molar masses uses camphor as the solvent. Camphor melts at 178.4 °C, and its large Kf (37.7 K kg mol-1) makes it especially useful for accurate work. A sample of an unknown substance that has a mass of 0.664 g reduces the freezing point of 25.0 g of camphor to 173.6 °C.
What is the molar mass of the unknown compound?

Answer 1

The melting point of camphor is 178.4°C which is also the freezing point.
After the unknown substance is added, the freezing point is reduced to 173.6°C.
The freezing point depression is
178.4°C - 173.6°C = 4.8°C
The molar mass of the substance can be found using the equation
Kf m = 4.8
37.7 (0.664 / MW) = 4.8
MW = 5.22 g/mol