Question

Calculate the molar mass of a compound if a solution of 12.0 g dissolved in 80.0 g of water freezes at −1.94 °C. Kf (freezing point depression constant for water) = 1.86 oC/m; FP (freezing point of water) = 0oC; density of water 1 g/mL

Answer 1

Molar mass = 144.2 g/mol

Step-by-step explanation:

Formula to solve this: ΔT = Kf . m

ΔT → Freezing point of pure solvent - Freezing point of solution

0°C - (-1.94°C) = 1.86 °C/m . m

1.94°C / 1.86 m/°C = m → 1.04 m

1.04 are the moles of solute in 1kg of solvent → molality (mol/kg)

Let's convert the mass of our solvent in kg to determine the moles.

80 g . 1kg / 1000 g = 0.08 kg

Molality . kg of solvent → Solute moles

1.04 mol/kg . 0.080 kg = 0.0832 moles

These are the moles of 12 g of solute. To find the molar mass → g/mol

12 g / 0.0832 mol = 144.2 g/mol