Question

In order to determine the molar mass of an unknown solute by measuring the freezing point depression of a solution of this unknown solute in a solvent as compared to the freezing point of the pure solvent (distilled water), we immersed a test tube with ~20ml of solute + distilled water in a ice-salt water bath, in addition to a test tube of ~20ml of pure distilled water (in a different ice-salt water bath) and recorded the temperature every 30 seconds for 12 minutes in order to determine the freezing point depressions. The following are the post lab questions. How would the molar mass determination of a strong electrolyte ionic compound by the freezing point depression procedure of this experiment be affected by each of the following scenarios?

a. The researcher assumed that a=1 at a concentration of 0.1 m.

b. The cooling curve for the solvent was analyzed correctly, but the minimum temperature of the supercooled region of the cooling curve was used to determine the freezing point of the solution.

c. The thermometer was incorrectly calibrated and consistently showed a temperature 0.08 degrees celcius higher than the actual temperature for all measurements.

d. A sample of potassium chloride is contaminated with a small amount of sodium bromide. (Assume that the measurements were taken under conditions where a=1 for both compounds).

e. The distilled water used in the experiments was contaminated with a very low concentration of magnesium chloride.

Answer 1

a.

The researcher assumed that a=1 at a concentration of 0.1 m. ​it will cause a decrease in the molar mass because he is taking a less than the required value. for electrolyte a >1

b.

The cooling curve for the solvent was analyzed correctly, but the minimum temperature of the supercooled region of the cooling curve was used to determine the freezing point of the solution​.​ it will cause a error in the molar mass because he is taking a wrong value of freezing point.

c.

The thermometer was incorrectly calibrated and consistently showed a temperature 0.08 degrees celcius higher than the actual temperature for all measurements. ​it will show a less value of freezing point depression or tempreture and therefore molar mass will be greater than expected.

d.

A sample of potassium chloride is contaminated with a small amount of sodium bromide. (Assume that the measurements were taken under conditions where a=1 for both compounds).​it will cause a decrease in the molar mass because he is taking a less than the required value. for electrolyte a >1

e.

The distilled water used in the experiments was contaminated with a very low concentration of magnesium chloride. it will cause a large decrease in freezing point and therefore molar mass will be less than expected.