Question

If you know that a sample of KI in water has a freezing point of -3.2C, what is the mass % of KI? What is the mole fraction of KI?

Answer 1

Solution :-

Lets find the molality of the solution using the delta Tf and Kf

Delta Tf = Kf* m *i

3.2 C = 1.86 C per m * m * 2

3.2 C / 1.86 C per m * 2 = m

0.8602 m = m

So the molality of the Ki is 0.8602 m

Means in the solution 1 kg water contains 0.86025 mol KI

Now lets calculate the mass of the KI

Mass = moles * molar mass

Mass of KI = 0.8602 mol * 166.028 g per mol

                   = 142.8 g KI

So the total mass of solution = 1000 g water + 142.8 g KI = 1142.8 g

% of the KI = (mass of KI/ mass of solution)*100%

                    = [142.8 g / 1142.8g) *100 5

                   = 12.5 %

So the percent of the KI = 12.5 %

Now lets calculate the mole fraction of the KI

Moles of KI= 0.8602 mol

Moles of water = 1000 g / 18.0148 g per mol = 55.56 mol

Mole fraction of KI = moles of KI/ total moles

                                  = 0.8602 mol / (55.56 mol + 0.8602 mol )

                                  = 0.0152

So the mole fraction of the KI = 0.0152