5 Steps to a 5 AP Chemistry

Comments

The experiment often uses hydrates of copper(II) sulfate, magnesium sulfate, calcium sulfate,

or barium chloride.

Experiment 3: Molar Mass by Vapor Density

Synopsis

The molar mass (molecular mass) of a volatile substance is determined in this experiment.

The mass of a sample of vapor is initially determined. This mass, along with the volume

of the container, the pressure, and the temperature, is used with the ideal gas equation to

calculate the molar mass. (See the chapter on Gases.)

Equipment

analytical balance barometer beaker(s)

clamp Erlenmeyer flask graduated cylinder

support stand

Measurements

1. the mass of the flask


2. the mass of the flask plus condensed vapor


3. the temperature of the water bath used to heat the flask


4. the barometric pressure


5. the number of milliliters of water required to fill the flask

Calculations

There are a variety of ways to do the calculations. Most of these, however, involve the

calculation of the number of moles (n) from the ideal gas equation: n=PV/RT. The mass

of the vapor sample is calculated from the difference between measurements 1 and 2. The

temperature (measurement 3) is converted to kelvin. The pressure (measurement 4) is

converted to atmospheres. Measurement 5 is converted to liters. Inserting the various num-

bers into the ideal gas equation allows you to calculate the number of moles. The molar

mass is calculated by dividing the mass of the sample by the moles.

Comments

Variations in this experiment usually combine the ideal gas equation with the mass of the

sample.

Experiment 4: Molar Mass by Freezing-Point Depression

Synopsis

The molar mass (molecular mass) of a solute is determined by measuring its effect on the

freezing point of a solvent. A cooling curve is constructed by plotting the temperature of a

solution that is slowly cooling versus time. After the solution completely freezes, the differ-

ence between the solution’s freezing point and the pure solvent’s freezing point is calculated.

The change in the freezing point is then related to the molality of the solution. (See Chapters 6

and 13.)

Experimental  283