# 4.R: Determination of the Molar Mass by Freezing Point Depression (Lab Report)

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## Lab Report: Determination of Molar Mass by Freezing Point Depression

Experimental Data

Unknown ID number: ____________________

Part 1: Freezing Point of Pure PDB
Mass of large empty test tube
Mass of test tube & PDB
Mass of PDB (by difference)
Part 2: Freezing Point of PDB-Unknown Mixture (~2 g)
Mass of vial & unknown
Mass of vial & unknown minus Sample I
Mass of unknown added (by difference)
Part 3: Freezing Point of PDB-Unknown Mixture (~4 g)
Mass of vial & unknown
Mass of vial & unknown minus Sample II
Mass of unknown added (by difference)

Temperature Measurements: Record the temperature every 30 seconds as the pure solvent and two solutions are cooled. Note the temperature at which any solid first appears.

Time Elapsed (minutes) Temperature (°C)
--- Pure Solvent (PDB only) Solution I (PDB + Sample I) Solution II (PDB + Sample II)
0
0.5
1.5
2
2.5
3
3.5
4
4.5
5
5.5
6
6.5
7
7.5
8
8.5
9
9.5
10
10.5
11
11.5
12
12.5
13
13.5

Graphical Analysis of Data

Use Excel to create three separate graphs of “Temperature versus Time” for the pure solvent and the two solutions studied. Each graph should have an appropriate title and labeled axes with an appropriate scale. Add two trendlines to the data points of each graph. You can do this by hand with a ruler or by using Excel. The first line is applied to data points that correspond to the cooling of the liquid stat: these are the points on the steep part of the graph. The second line is applied to data points that correspond to the co-existence of both the solid and liquid (freezing): these are the points on the part of the graph where the temperature levels out. Extrapolate the two trendlines towards each other until they intersect. The temperature at the point of intersection is the solvent freezing point and should be clearly shown on each graph. Attach your three graphs to this report.

Record the freezing point temperatures obtained from the graphs below:

• Pure PDB ________________°C
• Solution I _________________°C
• Solution II ________________°C

Calculation of Molar Mass

Complete the table below with the results of your calculations. Be sure to include all units. Note that $$K_{f}$$ PDB = 7.10 °C·kg·mol-1.

Solution I

Solution II

Freezing point of pure PDB

Freezing point of solution

Total Freezing point depression, $$\Delta T_{f}$$

Molality of solution

Moles of unknown in solution

Molar Mass of unknown

• Unknown number ______________________ has an average molar mass of _______________________g/mol.
• On a separate sheet of paper, show the equations used and calculations performed for molality of the solutions, moles of unknown in the solutions, and molar mass of the unknown. Attach your sheet to this report.

### Questions

1. Look up the freezing point of para-dichlorobenzene. This may be found in the CRC Handbook of Chemistry and Physics (library) or from various online sources.
• Freezing Point of PDB =_________________________
• Reference source used:
1. Using the freezing point from your reference source, determine the percentage error in your experimentally measured freezing point. Show your calculation below, and be sure to report your answer to the correct number of significant figures.
1. Suppose you find out that the actual molar mass of your unknown solid is exactly three times larger than the value you determined experimentally. What could you conclude about the nature of your unknown solid and the assumptions you made in your calculations in such a case? Explain.

4.R: Determination of the Molar Mass by Freezing Point Depression (Lab Report) is shared under a not declared license and was authored, remixed, and/or curated by Santa Monica College.