Question

Calcium Chloride (CaCl2) is often used in place of NaCl for melting snow because it is believed to provide greater freezing point depression. One theory is because it can dissociate to three ions instead of the two ions for NaCl, as shown below:

It is known that when NaCl dissolves in water, it fully dissociates into Na+ and Cl- ions. When CaCl2 dissolves in water, only 65% of the CaCl2 dissolves into Ca2+ and Cl- ions, and the rest dissolves simply as CaCl2 molecules in solution. What will be the freezing points for: a) 1 mole of NaCl in 100 moles of water (10 pts) b) 1 mole of CaCl2 in 100 moles of water (15 pts) Assume that pure water freezes at 0 °C, and the Heat of Fusion for water is 6012 J/mol.

Answers: Part a) T = 271.08 K Part b) T = 270.87 K

Answer 1

Ans :

a)

Mass of 100 moles of water = 100 x 18.01528 =1801.5 g = 1.8 kg

molality = moles / mass (kg)

NaCl gives 2 mol ions

= 2 / 1.8

= 1.11 m

Freezing point depression = kf x m

kf for water 1.86 k/m

= 1.86 x 1.11

= 2.07 K

Pure water freezes at 0 c = 273.15 K , so the freezing point of solution :

= 273.15 - 2.07

= 271.08 K

b)

Now 65% of 1 mol CaCl2 dissociates as 3 ions and rest 35% remains as CaCl2

So total mol of solute :

= 3[(65 x 1) / 100] + [(35 x 1) / 100]

= 2.3 mol

Molality = 2.3 / 1.8

= 1.28 m

Depression = 1.86 x 1.28

= 2.38 K

So freezing point of solution = 273.15 - 2.38

= 270.77 K