Practice Problems — Continued

2. (b) Calculate the ratio of the distance traveled by NH3(g) to distance traveled by HCl(g).

(c) Give two factors that must be carefully controlled to ensure the expected result.

3. If the average velocity of a N2 molecule is 475 m/s at 25°C, what is the average velocity of a He atom at 25°C?

The Effects of Volume and Intermolecular Forces on Real Gases

Measurement of the volume of one mole of most gases at STP results in a number that is near, but seldom exactly equal to 22.4 L. As pressure increases and temperature decreases, the volume of one mole of gas deviates even further from this value. In a previous section it was stated that gases behave most ideally when pressure is low and temperature is high. The reason for this is two-fold:

1. Real gases do have some volume, while ideal gases are assumed to have no volume and to exist as tiny points in space.

2. Real gases exert some attractive forces on their neighboring particles, while ideal gases are assumed to be completely independent of the gas particles that surround them.

In 1873, Johannes van der Waals modified the ideal gas law to fit the behavior of real gases. Van der Waals inserted two small correction factors. The first accounts for the volume of the gas particles themselves and the second corrects for the IMFs existing between gas particles. The Van der Waal’s equation is really just a more complex form of the ideal gas law.

The “correction factors” include an a factor and a b factor, which account for the attractive forces and the volume that real gases demonstrate. Every gas has its own unique values for a and b. Note that the units of a and b allow proper cancellation to produce units of pressure and volume respectively when substituted into the van der Waals equation.