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Laboratory Techniques I, CHY 114
Chemistry Department, University of Southern Maine

Colligative Properties : Measuring the Mass of a Molecule

Experiment created by the UMaine InterChemNet© Team. Adapted with permission.

Assignment

Use your classroom text to read about these subjects:

• colligative properties, particularly freezing-point depression
• using colligative properties to determine molar mass

(These topics are covered in section 12.6 of Chang, Chemistry, 9th edition.)

Use the index in your Laboratory Handbook to find out about care and use of thermometers.

Goals

In this experiment, you will learn how to

• plot cooling curves and use them to determine freezing points of liquids;
• measure the amount by which solutes depress the freezing point of pure liquid;
• use freezing-point depression to determine the molar mass of a substance.

Overview

Colligative properties are properties of a solution that depend on the concentration of solute particles, not on the chemical properties of the solute. These physical properties include lowering of vapor pressure, boiling-point elevation, osmotic pressure, and freezing-point depression.

Graphs of temperature versus time for a cooling liquid, known as cooling curves, are helpful in comparing the freezing points for solvents and solutions. You can determine the freezing point for a pure solvent like naphthalene (C₁₀H₈) from a cooling curve. Subsequently, from a cooling curve for a solution of naphthalene and a solute, you can measure the freezing-point depression of naphthalene caused by the solute. The relationship between the change in the freezing point (freezing-point depression) and the concentration of the solution is as follows:

t =K_fpm (Equation 1)

where t is the change in freezing point temperature in celcius degrees), K_fp is the molal freezing point depression constant, and m is the molal (not molar) concentration of the solution. Molality is the number of moles of solute per 1000g of solvent. Unlike molarity (the number of moles of solute per liter of solution), molality does not change with temperature. Hence molality is a convenient unit of concentration to use under conditions of changing temperature. By solving Equation 1 for m with t determined from a cooling curve, you can determine the molality of a solution, and from that, the number of moles of the solute present. If you previously weighed the solute, then you know both the mass (g) and the number of moles of the same sample. You can then determine its molar mass:

molar mass (g/mol) = [mass of sample (g)] / [# moles of same sample]

Preparing for Lab

The following problems require calculations similar to those called for in the report on this experiment. Learn how to work these problems, showing your calculations with units. For calculations, answers are provided. Similar questions may appear on your prelaboratory quiz. For more guidance, look at the Report Form for this experiment.

1. The freezing point of pure naphthalene (C₁₀H₈) is 80.6 °C. Polly Ester dissolves 0.907 g of a solid in 6.07 g of melted naphthalene. She finds that the freezing point of the resulting solution is 63.3 °C. Calculate the freezing point depression, t. ANSWER: 17.3 °C
2. For naphthalene, the freezing-point depression constant is 6.9 °C/m. Calculate the molality m of solute in the solution of question 1. ANSWER: 2.5 m
3. From your answer to question 2, calculate the number of moles of solute in the solution under study. ANSWER: 1.51 x 10^-2 mol
4. From your answer to question 3, calculate the molar mass of the solute. ANSWER: 60.0 g/mol

Procedures

Download and print the Procedure for this experiment. Bring it with you to lab

We recommend that you translate all procedures into an outline, with numbered steps to follow in lab. At the prelaboratory session, ask your lab instructor for further explanation of sections you cannot translate, or terms that you cannot find in your text or do not understand.

Report

Download and print the Report Form for this experiment. Bring it with you to lab. You will write data and observations on the form during lab, as you carry out the Procedure. After lab, you will complete the Report Form by carrying out calculations to give your final lab results. These calculations include graphing your data with Excel, so if you need to use USM computers for this purpose, be sure to allow time to visit the USM computer lab for this work.

CHY114 Lab Manual