5 Steps to a 5 AP Chemistry

the solvent) is used to calculate the change in temperature (ΔT) between the freezing point

of the solvent and the solution. The change in temperature divided by the freezing-point

depression constant (from a table) will give the molality of the solution. The molality of the

solution times the kilograms of solvent yields the moles of solute. Finally, the mass of

the solute divided by the moles of solute gives its molar mass.

Comments

One variation in this experiment is to add the solute to the test tube before the solvent.

Experiment 5: Molar Volume of a Gas

Synopsis

The volume occupied by a mole of a gas is calculated in this experiment. A sample of a solid

substance is heated, decomposing it into several products, including a gas. The mass of the

gas is determined by the weight difference of the solid before and after heating and is then

converted to moles. The volume of the gas, the pressure, and temperature are measured. (See

the chapter on Gases.)

Equipment

analytical balance barometer beaker(s)

clamp Erlenmeyer flask graduated cylinder

rubber tubing test tube(s) thermometer

Measurements

1.the barometric pressure

2.the mass of the test tube plus solid sample (before the reaction)

3.the mass of the test tube plus sample after the reaction

4.the temperature of the water

5.volume of water displaced into the beaker, if the variation is used

The mass of the test tube after the reaction must be determined after the test tube has com-

pletely cooled to room temperature.

Calculations

The temperature must be converted to kelvin (T 1 ), and the volume of water is normally

expressed in liters.

The mass of gas generated is calculated by taking the difference between measurements

2 and 3. Using the molar mass of the gas, the mass of gas is converted to moles of gas (n).

For the variation, the vapor pressure of water at the recorded temperature is found in a table.

The pressure of the gas (P 1 ) is the difference between the value in the table and measurement 1.

The volume of water in the beaker is the volume of the gas (V 1 ).

Calculate the volume (V 2 ) of the gas at STP (T 2 and P 2 ) using the combined gas law.

The molar volume of the gas is the volume at STP (V 2 ) divided by the moles of gas (n).

Comments

The most common procedure is to produce oxygen gas by decomposing KClO 3.

A common variation is to measure the volume of gas produced by displacing water from

a flask. The volume of water displaced is the volume of gas generated at that temperature

and pressure. From a measurement of the atmospheric pressure and the temperature of the

gas, the volume of gas at STP can be calculated.

Experimental  285