Question

Hi, I was wondering how to calculate the molar masses of these 2 compounds since my instructions say to use my data and not the periodic table. The data is also listed below. Thanks!

· Glycerol

· NaCl

· CaCl₂

_Data:

_{H2O freezing point: 0}

_{Glycerol freezing point: -1}

_{NaCl freezing point: -5}

_{CaCl2 freezing point: -4.3}

Answer 1

Apply colligative properties

Tf normal of water = H2O, the solvent = 0°C

so

dTf = -Kf*m*i

m = molality = mol of solute / kg solvent

i = vant hoff effect, i.e. number of ions

Glycerol, i = 1

NaCl = i = 2,

CaCl2 = i = 3

so...

a)

Glycerol

dTf = -Kf*m

for water, Kf = 1.86

so

-1 = 1.86 * mass of glycerol / MW of glycerol /(kg of solvent)

-5 = 1.86 * mass of NaCl/ MW of NaCl/(kg of solvent) * i

-4.3 = 1.86 * mass of CaCl2 / MW of CaCl2 /(kg of solvent) * i

Solving for Molar Mass:

MW of sample = Kf*mass of sample / kg of solvent / ( dTf )

MW of Glycerol = 1.86*mass of sample / kg of solvent / ( 1) * 1

MW of Glycerol = 1.86*mass of sample / kg of solvent / ( 5) * 2

MW of Glycerol = 1.86*mass of sample / kg of solvent / ( 4.3) * 3

Note that you will need:

- kg of solvent ( water ) used

- mass of sample used