Question

2. Sodium chloride, potassium chloride, and calcium chloride are all used to de-ice streets. given equal concentrations of each, which de-ices streets best? Explain why.

3. The freezing point of water decreases as salt is added until it reaches the limit of sodium chloride solubility at -21.0 C. What is the molality of the solution at this temperature?

Answer 1

Q2.

Apply Colligative properties

This is a typical example of colligative properties.

Recall that a solute ( non volatile ) can make a depression/increase in the freezing/boiling point via:

dTf = -Kf*molality * i

where:

Kf = freezing point constant for the SOLVENT; ENT;

molality = moles of SOLUTE / kg of SOLVENT

i = vant hoff coefficient, typically the total ion/molecular concentration.

At the end:

Tf mix = Tf solvent - dTf

therefore,

choose the one wtih more "i" factor, i.e. more ions,

NaCl , KCl, CaCl2

CaCl2 =3 ions, Ca2+ + 2Cl- will be the best

Q3

Tf mix = Tf solvent - dTf

-21 = 0 - i*Kf*m

21 = 0 + 2*1.86*(molality)

molality = 21/1.86/2

molality of NaCl = 5.6451