EXAMPLE LAB REPORT #2

Physical Chemistry
I.M. Chemist

Experiment #6
Lab Partner: L. Boltzmann

Molecular Weight by the Boiling Point Method

Laboratory Work Began Nov. 20, 1979

Laboratory Work Completed Nov. 27, 1979

Report Turned in December 4, 1979

I. Purpose

The purpose of this experiment is to determine the molecular weight of a non-volatile solute by the boiling point elevation method. The solvent is acetone and the solute is p-nitrotoluene.

II. Theoretical Discussion

The boiling point of a solvent is raised by the addition of a non-volatile solute due to the fact that the vapor pressure of the solvent is lowered by the impurity. The following equation is derived from the theory of ideal solutions:



DTb is the elevation of the boiling point, m is the molar concentration of the solute,

DH2 is the molar heat of vaporization of the solvent, M1 is the molecular weight of the solvent, R is the ideal gas constant, and T0 is the boiling point of the pure solvent.

References: Physical Chemistry, Moore, 1972, p. 276;

Physical Chemistry, Daniels and Alberty, 1967, p.153.

III. Experimental Procedure

A. Apparatus

1. Cottrell-Ramsay-Young boiling point apparatus

2. Beckman thermometer.

B. Procedure

  1. Obtain the boiling point of a known weight of acetone (approximately 50 ml).
  2. Add known weight of solute and note boiling point when it becomes constant.
  1. Repeat.

C. Reference: A Brief Introduction to the Physical Chemistry Laboratory, Myers, 1972, p. 48.

IV. Data and Calculations

A. Data

weight acetone = 39.6 g

boiling point of acetone = 2.790oC (on Beckman)

Run

Number

 Weight Solute

Added

 Total Weight

Solute

 Boiling Point Temperature

Elevation
1 0.4585 g 0.4585 g 2.950o 0.160o
2 0.2390 g 0.6979 g 3.025o 0.235o
3 0.3612 g 1.0591 g 3.155o 0.365o

B. Sample Calculation

where,

DTb = boiling point elevation

Kb = molal boiling point constant

W2 = grams of solute

W1 = grams of solvent

or

M2 = molecular weight of solute

Kb = 1.71 for acetone

W1 = 39.6 grams of acetone

= 124 

 

C. Results

Run Number Molecular Weight
1 124
2 128
3 125
Average 126.2 2
Correct Value 137


V. Discussion of Results

The molecular weight of p-nitrotoluene was found to be 126 g / mol. This represents an error of 8% as the correct value is 137.

Since precise instruments, such as the Beckman thermometer and Mettler balance, were used, it seems that the error should be considerably less than 8%. Some sources of error include reading instruments, impure chemicals, and error in procedure.

 
If you have questions, comments or suggestions, email me at jbentley@deltastate.edu Last updated:  December 15, 2007