Exp 8: DETERMINATION OF MOLAR MASS OF A SOLID FROM FREEZING POINT DEPRESSION--A COMPUTER ASSISTED EXPERIMENT

Introduction:

DELTA T = K_fm

where K_f is the molal freezing-point depression constant unique for each solvent.

In this experiment you will be given 1,4-dichlorobenzene, C₆H₄ Cl₂, as a solvent and a solid whose molar mass you will determine from the observed freezing point depression.

Procedure:

I. Freezing Point of the Solvent

Solvent Preparation: Obtain a small test tube (75 mm x 10 mm). Add about 150-250 mg of sample to the test tube and record its exact mass. (Use the 4-place balance.)

Place the tube in a 50 mL beaker half-full of boiling water. After the sample has melted remove the heat, insert the temperature probe (or thermister) into the sample, and clamp a thermometer into the beaker of hot water. Be careful not to allow water in the sample. Stir the sample gently with the thermister while watching the water temperature drop.

When the water (and sample) temperature has dropped to 65C, start sampling and graphing the data.

Analysis of Data:

Press the arrows to move the cursor along the lines to determine the point of intersection, which is the melting temperature of the solvent. Record this temperature. (Alternately, the freezing temperature can be observed directly from the graph by moving the cursor to that point and reading the temperature from the screen.)

II. Freezing Point of a Solution:

Sample Preparation:

Mass out 1.5-2.5 mg of solute to the nearest 0.1 mg into a small dry test tube. Place the the tube with the solvent and thermister back into the beaker of boiling water. Remove the thermister after the solvent melts and add the solute sample prepared above. Remove the heat and stir the solution gently with the thermister. When the water (and sample) temperature has dropped to 70C start sampling.

Follow procedures described above for sampling, saving and plotting the data.

Analysis of Data:

Use the cursor to determine when the solution first begins to freeze out and record this as the freezing point of the solution.

Calculations and Questions:

1. What is the freezing point depression , DELTA T, for the solution?

2. The freezing point depression constant, Kf , for 1,4- dichlorobenzene is 7.10C/m. Calculate the apparent molality of your solution.

3. How many kilograms of solvent were used?

4. What is the apparent number of moles of solute?

5. What is the apparent molecular weight of the solute ?

6. Why is the curve for the freezing of a solution different in slope from the freezing of a solvent?

7. What could cause the " dip " in the cooling curve?

8. The freezing point of 1, 4-dichlorobenzene is 53C. What is the % error in your result?

9. What are the natural random uncertainties in reading your thermometer and balance.

10. Compute the % relative error in each measurement and the % relative error in the molar mass of the solute. What is the absolute error in your molar mass? Express this error in plus/minus form with the proper number of significant digits.

11. Take the derivative of your cooling curves, dT/dt. How could you use calculus to find the freezing point of a solvent or solution? Students in calculus are expected to try it. Other students may try this if they wish.