Free Gases MCQs with Answers

11. Which assumption is NOT part of the kinetic molecular theory of an ideal gas?

• A. Gas molecules are in constant random motion
• B. Collisions between molecules are perfectly elastic
• C. There are strong attractive forces between gas molecules
• D. The volume of the molecules is negligible compared with the volume of the container

Explanation: An ideal gas is defined by the absence of intermolecular forces, which is precisely why it obeys the gas laws exactly. Real gases do have such forces, which is why they deviate at high pressure and low temperature. The other three assumptions are all part of the model.

12. The pressure exerted by a gas on the walls of its container is caused by

• A. the weight of the gas molecules
• B. collisions of the molecules with the walls
• C. attraction between molecules and the walls
• D. the chemical reaction of the gas with the walls

Explanation: Each collision delivers a small impulse, and the enormous number of collisions per second averages out into a steady pressure. Raising the temperature makes the molecules strike harder and more often, which is why pressure rises with temperature at constant volume. Weight contributes negligibly at ordinary scales.

13. At constant temperature, doubling the pressure on a fixed mass of gas will

• A. double its volume
• B. halve its volume
• C. leave the volume unchanged
• D. quadruple its volume

Explanation: Boyle's law states that pressure and volume are inversely proportional at constant temperature, so the product PV stays constant and doubling one halves the other. A graph of P against V is a hyperbola, while P against one over V is a straight line. This is why a syringe becomes hard to compress further as the volume shrinks.

14. A gas occupies 500 cm3 at 2 atm. At constant temperature, what volume will it occupy at 1 atm?

• A. 250 cm3
• B. 500 cm3
• C. 1000 cm3
• D. 1500 cm3

Explanation: Using P1V1 equals P2V2, the new volume is 2 multiplied by 500 divided by 1, which is 1000 cm3. Halving the pressure doubles the volume, so an answer smaller than the original must be wrong at a glance. Checking the direction of the change before calculating catches most errors.

15. Charles's law states that at constant pressure the volume of a fixed mass of gas is

• A. inversely proportional to the absolute temperature
• B. directly proportional to the absolute temperature
• C. independent of temperature
• D. proportional to the square of the temperature

Explanation: Volume rises in direct proportion to absolute temperature, so a graph of V against T in kelvin is a straight line that extrapolates back to the origin. Using degrees Celsius breaks the proportionality entirely, which is why the temperature must always be converted. This is the law behind a hot air balloon.

16. A gas occupies 300 cm3 at 27 degrees Celsius. At constant pressure, what volume will it occupy at 127 degrees Celsius?

• A. 400 cm3
• B. 1411 cm3
• C. 200 cm3
• D. 600 cm3

Explanation: Converting to kelvin gives 300 K and 400 K, and the volume scales in the same ratio, so 300 multiplied by 400 divided by 300 gives 400 cm3. Using Celsius directly would give the wrong ratio of 127 over 27 and the absurd second option. Absolute temperature is non negotiable in every gas law calculation.

17. Absolute zero is the temperature at which

• A. the volume of an ideal gas extrapolates to zero
• B. water freezes
• C. all gases liquefy
• D. pressure becomes infinite

Explanation: Extending the straight line of a volume against temperature graph backwards, it meets the axis at minus 273.15 degrees Celsius, which is defined as zero kelvin. No real gas actually reaches this point, since every gas liquefies first, so the zero volume is an extrapolation rather than an observation. It is the lowest temperature theoretically attainable.

18. The ideal gas equation is

• A. PV equals nRT
• B. PV equals nR divided by T
• C. P divided by V equals nRT
• D. PVT equals nR

Explanation: The equation combines Boyle's, Charles's and Avogadro's laws into one relation, with R the universal gas constant at 8.314 J per mole per kelvin. Every quantity must be in consistent units, and the temperature must always be absolute. Rearranging it gives the density and molar mass of a gas from measurable quantities.

19. Two moles of an ideal gas occupy what volume at standard temperature and pressure?

• A. 22.4 dm3
• B. 44.8 dm3
• C. 11.2 dm3
• D. 24 dm3

Explanation: One mole occupies 22.4 dm3 at STP, so two moles occupy twice that, which is 44.8 dm3. This follows from Avogadro's law, that equal volumes of gases contain equal numbers of molecules under the same conditions. The identity of the gas makes no difference, which is what students most often doubt.

20. Standard temperature and pressure are defined as

• A. 25 degrees Celsius and 1 atm
• B. 0 degrees Celsius and 1 atm
• C. 0 K and 1 atm
• D. 100 degrees Celsius and 1 atm

Explanation: STP is 273.15 K, that is 0 degrees Celsius, at one atmosphere, and it is under these conditions that a mole of gas occupies 22.4 dm3. Room temperature and pressure, at 25 degrees, is a different reference where the molar volume is about 24 dm3. Quoting the wrong reference changes every subsequent calculation.