Determination of Molar Mass by Colligative Properties Lab Report PDF

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Determination of Molar Mass by Colligative Properties Lab Report...

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Determina Determination tion of Mo Molar lar Mass by Colligative Properties A freezing P Point oint Depression Experiment

Name: Shaili Batsri Date: 09/07/2021 Lab Partners: Chris Zalayes

Introduction: Colligative properties of solutions are properties that depend upon the concentration of solute molecules or ions, but not upon the identity of the solute. Colligative properties include vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. When a solid is dissolved in a liquid, the freezing point of that liquid is depressed. It is possible to calculate the extent of the depression using the molality of the solution, and the constant which characterizes the solvent used, Kf. In this lab, we are going to determine the freezing point depression of t-butanol, and to calculate the molar mass of an unknown substance using the relationship between ΔTf and molality.

Procedure: Part I – Determination of Kf of T-But T-Butanol anol 1. Melt the t-butanol in a test tube using a warm water bath. 2. Using a temperature probe connected to the Sparkvue software create a graph of temperature vs. time and start to collect data. 3. Estimate the freezing point of t-butanol from the data collected. *Repeat until the most accurate data is collected. 4. Calculate the Molality and the ΔTf values. Use those values to calculate the K f value of tbutanol.

Part II – Determination of The Molar Mass o off An Unknown 1. Weigh a small sample from the unknown substance and melt it in a test tube containing tbutanol. 2. Using a temperature probe start to collect data by creating a temperature vs. time graph. 3. Estimate the freezing point of the solution of the t-butanol and the unknown from the data collected. *Repeat until the most accurate data is collected. 4. Using the relationship between ΔTf and molality, calculate the molar mass of the unknown substance.

Pre-Lab Questions: 1. Why is t-butanol used as a solvent? T-butanol is used as a solvent since its freezing temperature is near the room temperature (25.05oC) and it is easily observed with an ice bath.

2. Can tap water be used to determine Kf for tt-butanol -butanol in this experiment? Why or why not? Tap water cannot be used to determine the kf for t-butanol in this experiment. Tap water does not have the same freezing point as deionized water since it contains more substances than deionized water. Using tap water for this experiment will cause to a greater Kf value.

3. A st student udent added a small sam sample ple to her unknown to 25 mL of t-butanol and placed the stirred solution in an ice bath. The temperature of th thee solution dropped to zero without freezing. What was the problem? What should the stude student nt change about the next experiment? Since the freezing point of t-butanol is near room temperature (25.05oC), when the temperature of the solution dropped to zero the t-butanol was supposed to be well frozen. One of the possible problems can be poor mixing of the unknown sample with the tbutanol. In the next experiment, the student should make sure that the solution is properly stirred and mixed.

4. a. A student dissolved 0.666 mL of water into 0.030 kg of solvent. W What hat was th thee molality of the solution?

b. What is Kf for the solvent in part a if the freezin freezing g point of the pure solve solvent nt was -4. -4.63 630C and the freezing point of the solution in part a is -10.110C?

c. What is the molar mass of an unk unknown nown if 3.90 g o off the u unknown nknown were dissolved in 0.030 kg of fresh sample of the same solvent us used ed in p arts a and b to give a freezi freezing ng point 0 depression of 3.65 C?

5. A careless student didn’t bother to keep the test tube co containing ntaining the t-butanol sealed from the air during the experiment. Explain briefly how this mi might ght affect results. A possible error that may affect the experiment results is not sealing completely the test tube that contains the t-butanol. Since t-butanol is hygroscopic, it quickly absorbs moister from the air. Not sealing the test tube properly will cause adding a solute to a solvent. This will result in the lowering of the freezing point of the solvent, t-butanol. A change in a freezing point will cause an error in the molality value, kf value, and molar mass value, which eventually will cause an unprecise result of the whole experiment.

6. Why use a volumetric pipet instead of a graduated cylinder to measure the volumes o off water and unknown added to the t-but t-butanol anol solutions? We should use a volumetric pipet instead of a graduated cylinder since volumetric pipet is more accurate because its long size decreases error of misreading the meniscus. In addition, volumetric pipets are designed to measure specific volumes like 2 mL. since we are using small volumes in this experiment using a volumetric pipet will give us more precise results.

Date Sheet: Determination Of Kf Of T-Butanol 1. Record each of the freezing points you determined for pure t-butanol. 24.90C, 25.00C 2. What is the average of the freezing points that you determined for t-butanol in part I? 24.950C 3. Complete the following table for part II. Experiment mL H2O 1 2

0.30 0.30

Moles H2O 0.017 0.017

mL Mass t-butanol t-butanol 30 0.0237 30 0.0237

Solution Freezing Molality Point 0.717 18.8 0.717 18.6

o mL H2O: given data. o Moles H2O:

o mL t-butanol: given data. o Mass t-butanol:

o Solution Molality:

o Freezing Point: determined from the graph (temperature vs. time).

ΔT

Kf

6.15 6.35

- 8.577 - 8.856

o ΔT: Tf solution: 24.950C (calculated in question 2). Tf solvent: 18.8, 18.6 (determined from the graph temperature vs. time). Calculating ΔT: ΔTf1 = Tf (solution) – Tf1 (solvent) = 24.950C – 18.80C = 6.150C ΔTf2 = Tf (solution) – Tf2 (solvent) = 24.950C – 18.60C = 6.350C

o Kf :

4. Calculate the average Kf for t-butanol:

Determination Of the Molar Mass of An Unknown 1. Unknown letter: C 2. Complete the following table. Experiment 1 2 o

Solute volume (mL) 2.0 2.0

Solute density (g/mol) 0.80 0.80

Solute mass (g)

Mass t-butanol

Freezing Point

ΔT

Moles solute

Solute Mw

1.6 1.6

0.0237 0.0237

16.0 16.4

8.95 8.55

0.024 0.023

66.667 68.818

Solute Volume (unknown) (unknown):: given data.

o Solute Density: given data. o Solute Mass:

o Mass T-Butanol: calculated in part I. o Freezing Point: determined from the graph (temperature vs. time). o ΔT: Tf solution: 24.950C (calculated in part I, question 2). Tf solvent: 16.00C, 16.40C (determined from the graph temperature vs. time). Calculating ΔT: ΔTf1 = Tf (solution) – Tf1 (solvent) = 24.950C – 16.00C = 8.950C ΔTf2 = Tf (solution) – Tf2 (solvent) = 24.950C – 16.40C = 8.550C

o Moles Solute:

o Solute Mw:

3. Show how you calculated the K f of t-butanol using data from one of your experiments from the first data table (Part II). Calculated in part I, question 3.

4. Show how you calculated moles of solute in the second data table using data from one of your experiments from the second data table. Calculated in part II, q question uestion 2.

5. Show your calculations for the molar mass of the unknown solute. Calculated in part II, question 2.

6. What is the average molar mass of the unknown from all of your experiments?

Conclusion: In this lab, we examined the colligative property freezing point depression. The objectives of the experiment were to determine the freezing point depression of t-butanol and to calculate the molar mass of an unknown solution using the relationship between ΔT f and molality. The results showed that the average value of K f of t-butanol is -8.717oC•kg/mol, and the average molar mass of the unknown solution is 67.256 g/mol. A possible error that may affect the experiment results is not sealing completely the test tube that contains the t-butanol. Since t-butanol is hygroscopic, it quickly absorbs moister from the air. Not sealing the test tube properly will cause adding a solute to a solvent. This will result in the lowering of the freezing point of the solvent, t-butanol. In this lab I learned how to find a freezing point of a substance, to calculate molality, and to find a molar mass of an unknown. I learned about the connection between the change in temperature, molality, and the freezing point. In addition, I learned about colligative properties of solutions and that they are not dependent on the identity of the solute but upon the concentration of the solute ions and molecules....