Question

Freezing point depression experiment

1/what effect does the presence of a nonvolatile solute have upon the freezing point of a solution?

2/Explain the diffrence between molarity and molality. What are the units on each?

3/what is the molality of a solution that contains 5.2g of urea(molar mass = 60g/mol)in 250g of benzene(c6H6)?

Answer 1

Part A

The presence of a non-volatile solute causes the solvent to freeze at a lower temperature than normal. The magnitude of the freezing point depression is directly proportional to the molality of solute:
?Tf = Kf*m
Kf is called the freezing point depression constant of the solvent

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Part B

Molarity (M) = moles of solute / volume of solvent (L)
Molarity expresses the concentration as the number of moles of solute per unit-volume of solution. Concentration expressed by molarity tells how much solute is present in a given volume of solution. It does not tell how much pure solvent is present.

Molality (m) = moles of solute / weight of solvent (kg)
Molality expresses the concentration as the number of moles of solute per unit-weight of pure solvent. Concentration expressed by molality gives a definite ratio of solute molecules to solvent molecules.

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