How do you know that if the equilibrium mixture contains 36% by volume of ammonia the mole fraction of ammonia must be 0.36?

Equal volumes of gases contain equal number of moles so that 36% volume must contain 36/100 moles which equals 0.36.

How can you check that your answers to the mole fraction of the three gases are correct?

If we add the three mole fractionswe we've calculated they should add up to 1. The figures are 0.36 + 0.16 + 0.48, which do add up to 1.

What relationship are we using to calculate the partial pressures of the gases at equilibrium?

We're using the expression: partial pressure = mole fraction x total pressure

How can we check that the values we've calculated for the partial pressures are correct?

The partial pressures should add up to the total pressure. 72 + 32 + 96 does add up to the total pressure of 200 atmospheres.

How can we calculate the units for K_p?

Using the expression for K_p the units must be (atm)²/(atm)(atm)³.

(atm)² can be cancelled from the top and bottom of the expression so units for K_p are 1/atm² or atm^–2.

How can you help students learn how to carry out calculations such as this?

Showing a worked example provides a model for students to follow. Asking them to explain the steps involved to a fellow student helps both students.