Question

Calculate the molality for each of the following solutions. Then, calculate the freezing-point depression

?T_F = K_Fc_m

produced by each of the salts. (Assume the density of water is 1.00 g/mL and

K_F = 1.86

Answer 1

We will use the following relation for freezing point depression,

Delta Tf = m x Kf

m = molality = moles of solute / kg of solvent

moles = g of solute / molar mass

Given are,

b. 27 g of KCl in 1.8 L of water

molar mass of KCl = 74.55 g/mol

mass of water in kg = L of water x density of water

kg of water = 1.8 x 1.0

                   = 1.8 kg

Then,

moles of KCl = 27 / 74.55

                      = 0.362 moles

molality of KCl solution Cm = 0.362 / 1.8

                                      = 0.201 m

Because the KCl salt is an ionic compound that dissociate in water to yield two ions per formula unit of KCl thus, the actual concentration of the dissolved species in the saturated solution will be

Actual concentration = 2