CBSE Class 12 · 2011 · All India; also 2010 · SolutionsShort Answer

What mass of NaCl (molar mass = 58.5 g mol

^{- 1}

) must be dissolved in 65 g of water to lower the freezing point by 7.5°C? The freezing point depression constant, K

_{f}

, for water is 1.86 K kg mol

^{- 1}

. Assume the van't Hoff factor for NaCl is 1.87.

Answer: ≈ 8.2 g

Solution

With dissociation,

Δ T_{f} = i K_{f} m

m = \frac{Δ T _{f}}{i K _{f}} = \frac{7.5}{1.87 \times 1.86} = 2.156 mol kg^{- 1}

Moles of NaCl in 65 g water:

n = 2.156 \times 0.065 = 0.1402 mol

Mass:

w = 0.1402 \times 58.5

w = 8.2 g

CBSE Class 12 Board · 2011 · All India; also 2010 · Subjective