Q19 of 50 Page 1

What mass of NaCl must be dissolved in 65.0 g of water to lower the freezing point of water by 7.50 °C? The freezing point depression constant (Kf) for water is 1.86°C/m. Assume Van’t Hoff factor for NaCl is 1.87. (Molar mass of NaCl = 58.5 g)

We know depression in freezing point ΔTf =


Where Kf is the molal freezing point depression constant of solvent = 1.86 °C/m


Vant Hoff’s factor ‘i’ is there as the compound NaCl is ionic. Given, i = 1.87


W2 = weight of unknown molecular material.


W1 = weight of water given = 65gm.


ΔTf= change in freezing point = 7.5°C


M2 = molecular mass of NaCl = 58.5gm.


∴ΔTf =


So, W2 = = = 8.1992 gm.


So, 8.1992gm of NaCl must be dissolved in 65.0 g of water to lower the freezing point of water.


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