Free Real and Ideal Gases MCQs with Answers

1. Real gases deviate most from ideal behaviour at

• A. high temperature and low pressure
• B. low temperature and high pressure
• C. high temperature and high pressure
• D. standard temperature and pressure

Explanation: At high pressure the molecules are close enough that their own volume is no longer negligible, and at low temperature they move slowly enough for intermolecular attractions to matter. Both assumptions of the ideal model fail together under those conditions, which is also why gases liquefy there. A gas behaves most ideally when it is hot and dilute.

2. In the van der Waals equation, the constant a corrects for

• A. the volume occupied by the gas molecules
• B. the intermolecular forces of attraction
• C. the number of moles present
• D. the temperature of the gas

Explanation: The term involving a raises the measured pressure to what it would have been without attractions pulling molecules away from the wall, so a is larger for gases that are easily liquefied. The constant b is the excluded volume correction for the finite size of the molecules. Moles and temperature already appear explicitly in the equation.

3. The van der Waals equation corrects the ideal gas equation for

• A. the mass of the container
• B. the finite volume of the molecules and the attractive forces between them
• C. the speed of the molecules
• D. the colour of the gas

Explanation: The constant b subtracts the volume actually occupied by the molecules and the constant a adds back the pressure lost to intermolecular attraction, so the equation describes real gases far better. Larger values of a and b indicate a gas that departs further from ideality. As pressure falls towards zero the corrections vanish and the ideal equation is recovered.

4. A gas can be liquefied only if it is cooled below its

• A. boiling point
• B. critical temperature
• C. melting point
• D. triple point

Explanation: Above the critical temperature no amount of pressure will produce a liquid, because the molecules have too much kinetic energy for the attractive forces to hold them together. This is why oxygen and nitrogen must be cooled deeply before they can be liquefied while ammonia liquefies at room temperature under pressure. The pressure needed at that temperature is the critical pressure.

5. Which gas would be expected to behave most ideally at room temperature?

• A. Helium
• B. Ammonia
• C. Water vapour
• D. Carbon dioxide

Explanation: Helium is a small non polar monatomic gas with very weak dispersion forces and negligible molecular volume, so it comes closest to the ideal model. Ammonia and water vapour are strongly hydrogen bonded and deviate considerably. Carbon dioxide, being larger and more polarisable, also departs more than helium.

6. The compressibility factor Z equals PV over nRT. For an ideal gas its value is

• A. zero
• B. exactly 1 at all pressures
• C. always greater than 1
• D. always less than 1

Explanation: By definition an ideal gas satisfies PV equal to nRT exactly, so Z is one under every condition. For a real gas Z falls below one where attractive forces dominate and rises above one at high pressure where molecular volume dominates. Plotting Z against pressure is the standard way of displaying how far a gas departs from ideality.