Ideal Gas Equation

Ideal Gas Equation MDCAT MCQs with answers

15 past paper MCQs on Ideal Gas Equation, from the Gases unit of MDCAT Chemistry. The year each question appeared is shown where known; tap “Show answer” for the correct option and a short solution.

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1.The expression $PV=nRT$ represents the: (MDCAT 2020)

  1. Dalton's law
  2. Avogadro's law
  3. general gas equation
  4. van der Waals equation
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Correct answer: (c) general gas equation

It is the ideal or general gas equation, got by combining the laws of Boyle, Charles and Avogadro.

2.The correct ideal gas equation is: (MDCAT 2024)

  1. $qV=nRT$
  2. $PV=nRT$
  3. $PV=nRt$
  4. $PV^2=nRT$
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Correct answer: (b) $PV=nRT$

$PV=nRT$, with $T$ the absolute temperature.

3.When pressure is 1520 torr then density of oxygen gas will be: (SIBA MDCAT 2025)

  1. 64/RT
  2. 32/RT
  3. 16/RT
  4. 128/RT
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Correct answer: (a) 64/RT

$1520\,\mathrm{torr}$ is two atmospheres, and $d=PM/RT$ gives $2\times32/RT$.

4.Which equation is used to calculate concentration for n mole of an ideal gas? (SIBA MDCAT 2025)

  1. P/RT
  2. PV/RT
  3. CV/RT
  4. PM/RT
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Correct answer: (a) P/RT

Dividing $PV=nRT$ by $V$ gives the concentration $n/V$ as $P/RT$.

5.Keeping temperature constant, if pressure is increased, density of gas will (SZABMU MDCAT 2025)

  1. Increases
  2. decreases
  3. gets doubled
  4. remains same
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Correct answer: (a) Increases

Density is $PM/RT$, so at fixed temperature it rises with the pressure.

6.A gaseous mixture contains 9.6% NH3, 22.6% N2 and 67.8% H2 gases. If the total pressure is 50 atm, then the partial pressure of H2 is (UHS MDCAT 2023)

  1. 67.8 x100 / 50
  2. 50 x 100 / 100
  3. 67.8 x 50 / 100
  4. 67.8 + 50 / 100
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Correct answer: (c) 67.8 x 50 / 100

A partial pressure is the mole fraction times the total, that is $67.8\times50/100$.

7.If both temperature and volume of gas are doubled, the pressure: (NUMS MDCAT 2023)

  1. Cannot be predicted
  2. Is reduced to 1⁄2
  3. Remains unchanged
  4. Is doubled
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Correct answer: (c) Remains unchanged

$P=nRT/V$, and doubling both the temperature and the volume leaves the ratio unchanged.

8.Which one of the following gases has the lowest density under room conditions? (UHS MDCAT 2023)

  1. Neon
  2. Nitrogen
  3. Oxygen
  4. Fluorine
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Correct answer: (a) Neon

Density goes with molar mass at a given temperature and pressure, and neon has the smallest of these.

9.Which of the following represents equation for ideal gas? (NUMS MDCAT 2022)

  1. PV=nRT
  2. PT=nRv
  3. P=nRT
  4. T=npv/R
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Correct answer: (a) PV=nRT

The ideal gas equation is $PV=nRT$.

10.How should the condition be changed to prevent the value of a given gas from expanding when its mass is increased? (NUMS MDCAT 2022)

  1. Temperature is lowered & pressure is increased
  2. Temperature is increased & pressure is lowered
  3. Temperature & pressure both are lowered
  4. Temperature & pressure both are increased
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Correct answer: (a) Temperature is lowered & pressure is increased

More moles would push the volume up, so the temperature must be lowered and the pressure raised to hold it.

11.Which variable mentioned in ideal gas is assumed to be constant in other gas laws? (NUMS MDCAT 2022)

  1. Volume
  2. Temperature
  3. Pressure
  4. Number of moles
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Correct answer: (d) Number of moles

Boyle’s, Charles’s and the pressure law all hold for a fixed mass of gas.

12.How will the conditions be changed to prevent the volume of the given gas from expanding when its mass is increased? (PMC Practice 2021)

  1. Temperature and pressure is increased
  2. Temperature is lowered and pressure is decreased
  3. Temperature and pressure is decreased
  4. Temperature is lowered and pressure is increased
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Correct answer: (d) Temperature is lowered and pressure is increased

More moles would push the volume up, so the temperature must be lowered and the pressure raised.

13.273 mL of a gas at STP was taken to 27oC and 600 mm pressure. The final volume of the gas would be: (PMC Practice 2021)

  1. 273 mL
  2. 300 mL
  3. 380 mL
  4. 586 mL
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Correct answer: (c) 380 mL

$273\times(300/273)\times(760/600)$ gives about $380\,\mathrm{mL}$.

14.According to the general gas equation, density of an ideal gas depends upon: (PMC MDCAT 2020)

  1. Pressure
  2. Temperature
  3. Molar Mass
  4. All of these
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Correct answer: (d) All of these

$d=PM/RT$ carries the pressure, the temperature and the molar mass together.

15.Which of the following is the correct equation to calculate relative molecular mass of a gas. (UHS MDCAT 2018)

  1. M = mPR / VT
  2. M = PV / mRT
  3. M = mPRT / V
  4. M = mRT / PV
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Correct answer: (d) M = mRT / PV

From $PV=(m/M)RT$ it follows that $M=mRT/PV$.

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