<u>Answer:</u> The molality of solution is 0.122 m and molar mass of unknown substance is 128.16 g/mol
<u>Explanation:</u>
- To calculate the depression in freezing point, we use the equation:

where,
= depression in freezing point = 2.50°C
i = Vant hoff factor = 1 (For non-electrolytes)
= molal freezing point elevation constant = 20.5°C/m
m = molality of solution = ?
Putting values in above equation, we get:

- To calculate the mass of cyclohexane, we use the equation:

Density of cyclohexane = 0.779 g/mL
Volume of cyclohexane = 20.0 mL
Putting values in above equation, we get:

- To calculate the molar mass of substance for given molality, we use the equation:

where,
= Given mass of solute = 0.2436 g
= Molar mass of solute = ? g/mol
= Mass of solvent (cyclohexane) = 15.58 g
Molality of solution = 0.122 m
Putting values in above equation, we get:

Hence, the molality of solution is 0.122 m and molar mass of unknown substance is 128.16 g/mol