Look at the conversation below in which two students discuss gaseous equilibria.

[INSERT GE_EU_02_cc]

What misconceptions are revealed in this conversation?

Student B hasn't understood that the pressure doesn't depend on the nature of a gas, only on the amount of it in a container.

The mole fraction refers to the fraction of the number of moles of gas, not of the M_r.

We can use K_c for gases but it's often more convenient to use K_p.

Square brackets are used to represent concentrations in mol dm^–3. It isn't appropriate to use them in expressions for K_p.

How might you help students who have such misconceptions?

A good way is to get students to use an animation such as the Phet animation gas properties. You can ask them to compare the pressure exerted by heavy and light molecules and to see how the pressure depends upon the mole fraction of the gases.