Free Ideal Gas Equation MCQs with Answers

1. What volume is occupied by 0.5 mole of an ideal gas at STP?

• A. 5.6 dm3
• B. 11.2 dm3
• C. 22.4 dm3
• D. 44.8 dm3

Explanation: One mole of any ideal gas occupies 22.4 dm3 at STP, so half a mole occupies 11.2 dm3. The identity of the gas is irrelevant, which is the point of Avogadro's law. The value 5.6 dm3 would correspond to a quarter of a mole.

2. 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.

3. 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.

4. Dalton's law of partial pressures states that the total pressure of a mixture of gases equals

• A. the product of the individual pressures
• B. the sum of the partial pressures of the component gases
• C. the average of the individual pressures
• D. the pressure of the heaviest gas

Explanation: Because ideal gas molecules do not interact, each gas exerts the pressure it would exert alone in the same volume, and these simply add. The partial pressure of a component is its mole fraction multiplied by the total pressure. This is how the composition of air and of respiratory gas mixtures is described.

5. A fixed mass of gas at 2 atm and 300 K is heated to 600 K at constant volume. The new pressure is

• A. 1 atm
• B. 2 atm
• C. 4 atm
• D. 8 atm

Explanation: Pressure is proportional to absolute temperature at constant volume, so doubling the temperature in kelvin doubles the pressure to 4 atm. Both temperatures are already absolute here, which removes the usual conversion trap. This is why pressurised cylinders carry warnings about heat.

6. Value of R gas constant in J mol-1 K-1 is

• A. 8.314
• B. 62.4
• C. 0.821
• D. 62400

Explanation: The universal gas constant is 8.314 joules per mole per kelvin, and this is the value to use whenever pressure is in pascals and volume in cubic metres. The figure 0.0821 dm3 atm per mole per kelvin is the same constant expressed for pressure in atmospheres, which is why 0.821 appears as a distractor. Always match the value of R to the units of the other quantities in the problem.