Question

The normal boiling point of ethyl acetate is 77.06°C. A solution of 50 g of a nonvolatile solute in 150 g of ethyl acetate boils at 84.27°C. The molar mass of solute is ______.

(K_b for ethyl acetate is 2.77°C kg mol⁻¹)

Options

• 64 g mol⁻¹

• 256 g mol⁻¹

• 128 g mol⁻¹

• 100 g mol⁻¹

Answer 1

The normal boiling point of ethyl acetate is 77.06°C. A solution of 50 g of a nonvolatile solute in 150 g of ethyl acetate boils at 84.27°C. The molar mass of solute is 128 g mol⁻¹.

Explanation:

For the solution of nonvolatile solute and ethyl acetate,

ΔT_b = `"T"_"b" - "T"_"b"^0`

= 84.27°C − 77.06°C

= 7.21°C

= 7.21 K

K_b = 2.77°C kg mol⁻¹ = 2.77 K kg mol⁻¹

Now, using formula and substituting values,

M₂ = `(1000  "K"_"b" "W"_2)/(Δ"T"_"b" "W"_1)`

M₂ = `(1000  "g kg"^-1 xx 2.77  "K kg mol"^-1 xx 50  "g")/(7.21  "K" xx 150  "g")`

= 128 g mol⁻¹