Gas Laws: Ideal Gas Law and the Gas Constant

Concepts

Three important gas laws are listed below. In these equations, a, b and c each represent a constant.

Boyle's Law
For a constant amount of gas at a constant temperature, the product of the pressure and volume of the gas is a constant.

P V = b

Charles's Law
For a constant amount of gas at a constant pressure, the volume of the gas is directly proportional to the absolute temperature.

V / T = c

At a given temperature and pressure, equal volumes of gas contain equal numbers of moles.

V / n = a

In each of these laws, the identity of the gas is unimportant.

Intuitively, one expects that each of these laws is a special case of a more general law. That general law is called the Ideal Gas Law.

P V = n R T

The constant R is called the gas constant, and its physical significance will be explored in other experiments.

Experiment

Objectives

• Verify the applicability of the Ideal Gas Law.
• Determine the value of the gas constant.

The molar concentration, C, of a gas is defined to be C = n / V. Using this definition, the Ideal Gas Law can be rewritten as

P = n R T / V = R T C

In effect, the pressure of the gas should be directly proportional to both the molar concentration and the temperature.

The molar concentration of a gas can be measured experimentally by weighing the amount of gas trapped in a flask or bulb with a known volume. The number of moles of trapped gas is obtained by dividing the mass by the formula weight. This technique is employed in this experiment to measure the molar concentration of nitrogen at various pressures and temperatures.

To perform the experiment:

1. Select a temperature. (Keep this temperature constant through Steps 2 - 8.)
2. Add nitrogen gas to the bulb.
3. Measure the pressure and mass of the gas.
4. Use the volume and mass to calculate the molar concentration of the gas.
5. Plot the pressure vs the molar concentration on the graph.
6. Repeat Steps 2-5 several times using different amounts of gas.
There should be multiple points on the graph.
7. Examine the plot to determine if the Ideal Gas Law is obeyed.
Do the points lie on a straight line?
8. Use the slope of the plot to determine the gas constant in units of L atm mole-1 K-1
9. Repeat the entire experiment at a different temperature.
Do you obtain the same value for the gas constant?

The glass bulb has a volume of 2.00 L.

The formula weight of N2 is 28.0134

1 atm   =   760 torr (by definition)

Click on the balance tare field or press the letter t on the keyboard to zero the balance.

oC

torr

mole/L

Concentration
( mole/L )

Pressure
( torr )

Pressure vs Concentration
torr L / mole

torr

GasConstant.html version 3.0
© 2000, 2014, 2023 David N. Blauch