Question

A sample of camphor used in the Rast method of determining molecular mass had a melting point of 176.5 oC. The melting point of a solution containing 0.522 g camphor and 0.0386 g of an unknown substance was 158.8 oC. Find the molar mass of the substance. (Kfp = 37.7 oC/mol)

Answer 1

Given info : A sample of camphor used in the Rast method of determining molecular mass had a melting point of 176.5°C. The melting point of a solution containing 0.522 g camphor and 0.0386 g of an unknown substance was 158.8 °C.

To find : the molar mass of the substance is ...

solution : mass of the unknown substance = 0.0386g

let the molar mass of the unknown substance = M

so no of moles of unknown substance = 0.0386/M

mass of the solvent ( camphor) = 0.522g

so the molality of the solution = no of moles of solute( unknown substance)/mass of solvent in kg

= (0.0386/M)/(0.522/1000) = 38.6/0.522M = 73.95/M

using formula, $\Delta T_b=K_b\times molality$

⇒ (176.5°C - 158.8°C) = 37.7°C/molal × 73.95/M

⇒ M = (37.7 × 73.95)/17.7 = 157.5 g/mol

therefore the molar mass of the substance is 157/5 g/mol