Question

What is the value of K_f if 30 g urea (molar mass 60) dissolved in 0.5 dm³ of water decreases freezing point by 0.15 K?

Options

• 0.30 K kg mol^-1

• 0.030 K kg mol^-1

• 0.15 K kg mol^-1

• 0.015 K kg mol^-1

Answer 1

0.15 K kg mol^-1 

Explanation:

Mass of water = 0.5 dm³ = 0.5 kg

Moles of urea = `30/60` = 0.5 mole

Molality of urea = `(0.5 "mol")/(0.5 "kg")` = 1 mol/kg

Δ T_f = K_f × m

`"K"_"f" = (Delta "T"_"f")/"m" = (0.15 "K")/(1 "mol"//"kg")`

= 0.15 K mol^-1 kg