Experiment 1: Determining Molar Mass From Freezing Point The objective of this e

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Experiment 1: Determining Molar Mass From Freezing Point

The objective of this experiment is to determine the molar mass of a known substance using its freezing point depression in a solution. You will be given three solutions in addition to the standard solution (water). The freezing point of water will be determined and then the freezing points of three other solutions will be determined. These solutions used in this lab are composed of sodium chloride (NaCl), calcium chloride (CaCl2), and glycerol. The freezing point data will be used to determine the molar mass, calculated using stoichiometry.

Materials:

600 mL Beaker
4 Glass test tubes
100 mL Graduated cylinder
10 mL Graduated cylinder
1-Hole Rubber stopper
Thermometer
100 mL Beaker
5 mL Glycerol, C3H8O3
2 Tbsp. Sodium chloride, NaCl
2 g Calcium chloride, CaCl2

Scale
Test tube rack
2 Weigh boats
Stir rod
Permanent marker
*Cubed/crushed ice
*Water
*75 mL Distilled water

*You must provide

Procedure

Label each of the weigh boats as either NaCl or CaCl2.

Place the NaCl weigh boat on the scale. Tare the scale.

Add approximately 2.0 g of sodium chloride to the weigh boat. Reweigh until you have obtained the correct amount of NaCl, and set the weigh boat aside.

Repeat Steps 2 - 3 to weigh the CaCl2.

Assemble the freezing point apparatus by getting a 600 mL beaker and filling it ¾ full of ice. Then, pour the remaining sodium chloride in your package (not the 2 g you weighed out in Step 3) in it and mixing it thoroughly.

Place the thermometer in the ice bath and monitor. The ice bath is ready when the temperature drops to approximately -10 °C.

While you wait for the temperature to drop, label the test tubes 1 – 4.

Use a 100 mL graduated cylinder to measure and pour 15 mL of water into test tube 1.

After the temperature in the ice bath has reached -10 °C, remove the thermometer from the beaker and rinse it with distilled water. Then, press a 1- hole rubber stopper into the top of the test tube to create a closed system. Place the thermometer in the opening, making sure that the thermometer is positioned in the water but not touching the bottom of the test tube.

Place the test tube in the ice bath, and record the freezing point for the water.
See Figure 3.

Next, use the 10 mL graduated cylinder to measure and pour 5 mL of glycerol into a 100 mL beaker.

Use the 100 mL graduated cylinder to measure and pour 25.0 mL of distilled water into the beaker to dilute the glycerol. Gently stir the solution with

Pour the solution into test tube 2.

Remove the thermometer from test tube 1 remold it on test tube 2. Place test tube 2 in the ice bath, and record the freezing point in Table 1.

Rinse out the 100 mL beaker and the thermometer with distilled water. Measure 25 mL of distilled water into the 100 mL beaker.

Add the 2.0 g NaCl aliquot that you measured in Step 3, and stir the solution to ensure an even distribution.

Pour the solution into test tube 3.

Repeat Step 14 to determine the freezing point of the NaCl solution.

Repeat Steps 15 - 18 to determine the freezing point of the CaCl2 solution.

Once these temperatures have been determined, find the molar mass of all the substances used, and compare to the theoretical values for accuracy.

Table 1: Freezing Point Temperatures Solution Freezing Point (*C)

H2O 0

Glycerol -5.3

NaCl -7.5

CaCl2 -4.0

Calculate the Molar Mass of Each Compound

~Glycerol m=dT/-Kf = -5.3/-1.86 = 2.849

~NaCl m=dT/-Kf = -7.5/((2*)(-1.86)) = 2.016

~CaCl2 m=dT/-Kf = -4/ ((3)(-1.86)) = 0.7168

QUESTION: How do each of these molar masses compare to the theoretical? What is the percent error?

Materials:

600 mL Beaker
4 Glass test tubes
100 mL Graduated cylinder
10 mL Graduated cylinder
1-Hole Rubber stopper
Thermometer
100 mL Beaker
5 mL Glycerol, C3H8O3
2 Tbsp. Sodium chloride, NaCl
2 g Calcium chloride, CaCl2

Scale
Test tube rack
2 Weigh boats
Stir rod
Permanent marker
*Cubed/crushed ice
*Water
*75 mL Distilled water

*You must provide

Explanation / Answer

delta T = Kf x i x M

M = molality

-5.3 = -1.86 x 1 x M

M = 2.85 which means 2.85 moles in 1 Kg or 1000 g

we have added 5 mL of glycerol which is 5 x 1.26 g/cc = 6.3 g in 25 g of water so in 1 Kg it is 6.3/25 x 1000 = 252 g

252 g is 2.85 moles then 1 mole is 252/2.85 = 88.4 g/mol is the mW of glycerol

2) NaCl

-7.5 = -1.86 x 2 x M

M = 2.016 which means 2.016 moles in 1 Kg or 1000 g

we have added 2 g of NaCl in 25 g of water so in 1 Kg it is 2/25 x 1000 = 80 g

80 g is 2.016 moles then 1 mole is 80/2.016 = 39.68 g/mol is the mW of NaCl

3) CaCl2

-4 = -1.86 x 3 x M

M = 0.717 which means 0.717 moles in 1 Kg or 1000 g

we have added 2 g of CaCl2 in 25 g of water so in 1 Kg it is 2/25 x 1000 = 80 g

80 g is 0.717 moles then 1 mole is 80/0.717 = 111.6 g/mol is the mW of CaCl2

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