Question

MOLECULAR MASS BY FREEZING POINT DEPRESSION

SHOW YOUR WORK PLEASE!

Procedures:

Experiment 1: Measure the Freezing Point of Pure Water

Take a clean test tube from the Containers shelf and place it on the workbench.

Take a balance from the Instruments shelf and place it on the workbench.

Place the test tube on the balance. Zero the balance.

Take water from the Materials shelf and add 10 mL to the test tube.
NOTE: In a classroom laboratory, you would remove the container from the balance before you add any chemicals or solutions. Filling the container separately prevents any spills, sprays, or splashes that might affect your measurements when they land on the balance. Luckily virtual labs are spill-, spray-, and splash-free, so you can skip that step to save time.

Record the mass of the water in your Lab Notes.

Take a thermometer from the Instruments shelf and attach it to the test tube.

Take a constant temperature bath and place it on the workbench. Run the bath at -15 °C.

Move the test tube into the constant temperature bath. Make sure the test tube is submerged in water.

Watch the temperature of the water in the test tube decrease. The water should begin to freeze. You will see some solid ice form in the test tube. You may want to use the + or – zoom buttons in the lower right of the screen for a closer view.

Record the first temperature when ice just begins to appear as the freezing point of the solvent in your Lab Notes.

Clear your station by dragging the test tube and thermometer to the recycling bin beneath the workbench.

Experiment 2: Measure the Freezing Point of a Solution of an Unknown Substance

Take a clean test tube from the Containers shelf and place it onto the workbench.

Move the test tube onto the balance. Zero the balance.

Take a thermometer from the Instruments shelf and attach it to the test tube.

Take FP sample 1 from the Materials shelf and add 2 g to the test tube. Record the mass of sample added in your Lab Notes.

Take water and add 10 mL to the test tube. Record the total mass of water and sample in your Lab Notes.

Move the test tube into the constant temperature bath. Make sure it is still set to -15 °C.

Record the freezing point of the solution as the first temperature when ice just begins to appear in your Lab Notes.

Clear your station by dragging the test tube to the recycling bin.

Repeat steps 1 – 7 but using unknown FP Sample 2.

Clear your station by dragging any containers and instruments to the recycling bin. Remember to press Save Notes.

NOTES:

Experiment 1

1. Mass of water was 10 g

2. 16, 11.3, 7.3, 3.9, 1.0, 0.0 degrees celsius is where it froze

Experiment 2

1. 2 g of sample fp 1 added

2. 12 g after water added to sample

3. -2.2 degrees when ice first appeared.

4. 2 g of sample fp 2 added

5. 12 g after water added to sample

6. -3.6 degrees when ice first appeared.

QUESTIONS

1. Experiment 1: Measure the Freezing Point of Pure Water
How many mL of water did you add to the test tube?

A. 10

B. 35 mL

C. 20 mL

d. 50 mL

2. Experiment 1: Measure the Freezing Point of Pure Water

At what temperature did the water in the test tube begin to turn into ice?

a. -2.0 degrees celsius

b. -15.0 degrees C

c. 10.0 degrees C

d. 0.0 degrees C

3. Experiment 1: Measure the Freezing Point of Pure Water

What does the freezing point measured for water tell you about the water's purity?

a. the water is not pure because the freezing point is lower than expected

b. the water is not pure because the freezing point is higher than expected

c. the water is not pure but the thermostat is not very accurate

d. the water is pure because the freezing point is exactly as expected

4. Experiment 2: Measure the Freezing Point of a Solution of an Unknown Substance

How many grams of FP sample #1 did you add to the test tube?

a. 12 g

b. 10 g

c. 8 g

d. 2 g

5. Experiment 2: Measure the Freezing Point of a Solution of an Unknown Substance

At what temperature did the water with FP sample #1 begin to freeze? Choose the closest answer.

a. 10 C

b. -2.1 C

c. 0.0 C

d. -5.0 C

6. Experiment 2: Measure the Freezing Point of a Solution of an Unknown Substance

What is the molality of FP sample #1? Choose the closest answer.

a. 0.56 mol/kg

b. 5.34 mol/kg

c. 1.13 mol/kg

d. 0.972 mol/kg

7. Experiment 2: Measure the Freezing Point of a Solution of an Unknown Substance
What is the molality of FP sample #2? Choose the closest answer.

a. 0.967 mol/kg

b. 9.67 mol/kg

c. 2.78 mol/kg

d. 1.94 mol/kg

8. What was the purpose of measuring the freezing point of water?

to assess the volume of water necessary to form ice which would be used for the solution of unknowns

b. to verify the accuracy of the apparatus and to determine the freezing point of the pure solvent

c. to determine the heat of the fusion of water which would be used in calculating the molar masses of the unknowns

d. to calculate the density of water versus ice

9. Suppose that the test tube you used for the experiment was not fully dry and there were some water droplets in the test tube. What effect would this have on the calculated molar mass of the unknown?

a. the calculated molar mass would be unaffected by the additional water

b. the calculated molar mass does not depend on the amount of solvent

c. the calculated molar mass would be much smaller than the true molar mass

d. the calculated molar mass would be larger than the true molar mass.

10. Suppose you added 4.000 g of FP sample #1 instead of 2.000 g, what would happen to the freezing point temperature of the water?

a. there is not enough info to calculate the freezing point depression

b. the freezing point would be depressed twice as much

c. the freezing point would be depressed half as much

d. the freezing poing would be increased twice as much

11. What is meant by "freezing point depression"?

a. when a solute and solvent are mixed together, the specific heat of the solution is depressed.

b. when a solute is added to a solvent, the freezing point of the solvent is lowered

c. when a solute is added to a solvent, the freezing point of the solvent is increased

d. the solubility of a solute changes as the solvent is frozen.

12. What is the first experimental step in calculating the molar mass by using the freezing point depression method?

a. you must first determine the molar mass of the solute by looking up the elements in the periodic table

b. you must first measure the heat of fusion of the solvent

c. you must first calculate the number of moles in a kilogram of solvent to determine its molality

d. you must first measure the freezing point of the pure solvent and then measure the freezing point of the solution

13. What is the molality of a solution?

a. the numver of moles of solute in a liter of solvent

b. the number of moles of solvent in a kilogram of solute

c. the number of grams of solute in a liter of solvent

d. the number of moles of solute in a kilogram of solvent

14.

Suppose you performed a similar experiment using benzene as your solvent. Given the data in the table below, what would be the temperature at which the benzene solution would begin to freeze? Choose the closest answer.

K_f of benzne 5.12 C/m

freezing point of pure benzene 5.48 C

molality of solution 0.724 mol/kg

i 1

a. 1.77 C

b. 3.71 C

c. 5.48 C

d. 9.19 C

15.

Which of the compounds in the table below is most likely to be your unknown FP sample 1?

Name of compound: Molar mass (g/mol)

Calcium sulfate 136.14

glucose 180.16

potassium hydrogen phthalate 204.22

sodium oxalate 134.00

a. glucose

b. calcium sulfate

c. sodium oxalate

d. potassium hydrogen phthalate

16. Suppose you dissolve 154.286 g of sodium chloride in 2.00 L of water. What is the molality of the solution given that the molar mass of sodium chloride is 58.44 g/mol and the density of water is 1.000 g/mL?

a. 2.64 mol/kg

b. 1.32 mol/kg

c. 771.1 mol/kg

d. 5.84 mol/kg

Answer 1