Lab 8 – Molar Mass

Lab 8 – Molar Mass

Name: [First and Last]

Date: DD/MM/YYY

Experiment #: 8

Title:  Molar Mass

Purpose:

The major objective of this experiment was determining the molar mass of an unknown solute by measuring the freezing point depression of cyclohexane caused by the addition of the solute. By comparing the estimated molar mass of the solute with the actual value, one will learn about colligative properties and how they can be used to calculate the molecular weights of substances.

Procedure:

1. Cyclohexane

• Measure 10.0ml of cyclohexane and put it in a test tube
• Fit the test tube with the special stopper at the top and suspend the mixing wire and the thermometer into the test tube
• Put the test tube into the beaker containing solute and the ice
• Using the mixing wire, stir the cyclohexane while observing the temperature changes
• Remove the test tube from the beaker at the freezing point of cyclohexane and record the temperature of cyclohexane after 30 seconds

1. Cyclohexane + Unknown Solute

• Measure 0.3014 grams of the solute and add it to the test tube containing cyclohexane (solvent)
• Swirl till the solute dissolves.
• After dissolving, dip the thermometer and the mixing wire into the test tube
• Put the test tube into the beaker containing the ice
• Mix the solution using the mixing wire while observing the temperature changes
• Remove the test tube from the beaker at the freezing point of the solution (Cyclohexane + unknown solute) and record the temperature (freezing point)
• Record the temperature after 30 seconds

Data/Results/Calculations:

1. Temperature Difference

The freezing temperature of pure cyclohexane = 6.6^oC

The freezing temperature of cyclohexane + unknown solute = 0.8^o C

Freezing point difference = 6.6^oC – 0.8^oC = 5.8^oC

1. Cyclohexane + Solute Warming Curve

10.0mL cyclohexane (0.7726g/mol) = 0.00773kg

Temp. Change = k_f   × m

5.8 ^o C = 20.4 ^o C/m × m

m = 0. 2843m

m = mol solute/kg solvent = g/g/m/kg solvent

1. 2843m = 0.3014m/ g/mol/0.00773kg = 137.0g/mol
2. Unknown Solute

Para- Dichlorobenzene

Actual molar mass = 147g/mol

Experimental molar mass = 137g/mol

% error = actual molar mass – experimental molar mass/ actual × 100 %

147g/mol – 137g/mol/ 147g/mol × 100%

= 6.8 %

Conclusions:

Through this experiment, I learned about the concept of colligative properties and how they can be utilized to calculate the molecular weights of substances. Additionally, measuring the freezing point depression of cyclohexane caused by the unknown solute made it possible to estimate its molar mass. The purpose of the experiment was successfully achieved, providing one with a practical understanding of colligative properties and their applications in determining molecular weights.

Notes:

All particles, regardless of identity, impact the solvent equally due to their interactions at the molecular level. The solvent’s properties, such as polarity and intermolecular forces, determine how particles dissolve and disperse, leading to consistent effects regardless of particle identity.

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This lab will be conducted by an experienced instructor. As you watch the lab, you should keep a lab notebook just as you would if you were personally conducting the lab. A well-kept lab notebook is the key to successful labs. The lab notebook acts as the record of your experiment, help you organize your thoughts and understand the results of the experiment, and will be useful in helping you write your lab report. Your lab notebook and/or lab report can be used as you take the lab exam that accompanies each experiment.

To complete the lab report requirement, be sure to follow the “Lab Report Sample” as your model and type out your lab report using the “Lab Report Template”. These can be found under “Lab Overview”. You can upload your report by clicking “Submit,” and then attaching your document.