Freezing Point Depression

How much NaCl must be dissolved in 4.00 kg of water to lower FP to -10.00 oC?.

Solution

Conc req’d = 5.38 molal

We need 5.38 mol of dissolved particles per kg of solvent.

NaCl(aq) --> Na+(aq) + Cl-(aq)

To get 5.38 mol/kg of particles we need

5.38 mol / 2 = 2.69 mol NaCl / kg

2.69 mol NaCl / kg ---> 157 g NaCl / kg

(157 g NaCl / kg)•(4.00 kg) = 629 g NaCl

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