Question

The addition of 5.00 g of a compound to 250g of naphthalene lowered the freezing point of the solvent by 0.780 K. Calculate the molar mass of the compound. [Kf (naphthalene) = 6.94 K kg mol−1 ]

Answer 1

we have below equation to be used:

delta Tf = Kf*mb

0.78 = 6.94 *mb

mb = 0.1124 molal

mass of solvent = 250 g

= 0.250 kg [using conversion 1 Kg = 1000 g]

we have below equation to be used:

number of mol,

n = Molality * mass of solvent in Kg

= (0.1124 mol/Kg)*(0.250 Kg)

= 2.81*10^-2 mol

mass of solute = 5.00 g

we have below equation to be used:

number of mol = mass / molar mass

2.81*10^-2 mol = (5.0 g)/molar mass

molar mass = 178 g/mol

Answer: 178 g/mol