Molar Mass via fp depression
1. A 200-mg sample of a purified compound of unknown molar mass is dissolved in 25.0 ml of benzene (d= 0.88 g/ml)
The resulting solution freezes at 2.6 oC. The freezing point of pure benzene is 5.5 oC and it freezing point constant
is 4.9. Determine the molecular mass of the unknown.
2. A 2.00-g sample of a large biomolecule was dissolved in 15.0 g of carbon tetrachloride. The boiling point of this
solution was 77.85 oC. Calculate the molar mass of the biomolecule. For carbon tetrachloride, the boiling point
constant is 5.03 oC .kg/mole, and the boiling point of pure carbon tetrachloride is 76.50 oC
3. Thyroxine, an important hormone that controls the rate of metabolism in the body, can be isolated from the thyroid gland.
When 0.455 g of thyroxine is dissolved in 100 g. of benzene, the freezing point depressed by 0.300 oC. What is the
molar mass of thyroxine? FP = 5.45 0C and Kf = 5.07 0C/m for benzene.
4. A 0.350-g sample of a large biomolecule was dissolved in 15.0 g of chloroform, and the freezing-point depression
was determined to be 0.240 oC. Calculate the molar mass of the biomolecule (Kf for chloroform is 4.70 oC kg/mol)