FSc Pre-Medical Chemistry Gases — Set 2

1. Question 1

   Q1. A sample of oxygen gas occupies 2.5 L at 1.2 atm. What is its volume at 0.8 atm?

   • A) 3.75 L
   • B) 1.67 L
   • C) 2.25 L
   • D) 4.0 L

   Answer: 3.75 L

   Explanation: Boyle's Law applied: P1V1 = P2V2, so V2 = 3.75 L. Option B fails due to incorrect calculation.

2. Question 2

   Q2. At STP, 1 mole of an ideal gas occupies how many liters?

   • A) 11.2
   • B) 22.4
   • C) 24.0
   • D) 20.0

   Answer: 22.4

   Explanation: Molar volume at STP is 22.4 L. Option C is incorrect as it's not STP volume.

3. Question 3

   Q3. A mixture of gases contains 2 moles of N2 and 3 moles of O2. What is the partial pressure of N2 if total pressure is 5 atm?

   • A) 1 atm
   • B) 2 atm
   • C) 3 atm
   • D) 4 atm

   Answer: 2 atm

   Explanation: Partial pressure = mole fraction * total pressure. N2 mole fraction is 2/5, so P(N2) = 2 atm. Option A fails due to wrong mole fraction.

4. Question 4

   Q4. A gas cylinder contains 10 kg of nitrogen at 20°C. If the cylinder's volume is 50 L, what is the pressure?

   • A) 140 atm
   • B) 170 atm
   • C) 200 atm
   • D) 280 atm

   Answer: 170 atm

   Explanation: Ideal Gas Law PV = nRT applied: n = 10 kg / 28 g/mol = 357.14 mol, P = 170 atm. Option A is incorrect due to miscalculation.

5. Question 5

   Q5. At what temperature (in Kelvin) will the average kinetic energy of gas molecules be double that at 300 K?

   • A) 300 K
   • B) 600 K
   • C) 900 K
   • D) 1200 K

   Answer: 600 K

   Explanation: Average kinetic energy ∝ T. So, if energy doubles, T must double to 600 K. Option A is incorrect as it doesn't change.

6. Question 6

   Q6. A sample of gas has a volume of 1.5 L at 25°C. What will be its volume at 50°C if pressure remains constant?

   • A) 1.63 L
   • B) 1.5 L
   • C) 1.35 L
   • D) 1.2 L

   Answer: 1.63 L

   Explanation: Charles' Law: V1/T1 = V2/T2. So, V2 = 1.63 L. Option B fails as it ignores temperature change.

7. Question 7

   Q7. The density of an ideal gas at 1 atm and 273 K is 1.25 g/L. What is its molar mass?

   • A) 28 g/mol
   • B) 32 g/mol
   • C) 40 g/mol
   • D) 44 g/mol

   Answer: 28 g/mol

   Explanation: M = dRT/P = 28 g/mol. Option B is incorrect molar mass for a different gas.

8. Question 8

   Q8. What is the root mean square speed of N2 molecules at 300 K?

   • A) 400 m/s
   • B) 516 m/s
   • C) 600 m/s
   • D) 800 m/s

   Answer: 516 m/s

   Explanation: urms = √(3RT/M) = 516 m/s for N2. Option A is too low due to incorrect calculation.

9. Question 9

   Q9. A gas is collected over water at 25°C and 1 atm. If the vapor pressure of water is 23.8 mmHg, what is the partial pressure of the gas?

   • A) 736.2 mmHg
   • B) 760 mmHg
   • C) 783.8 mmHg
   • D) 800 mmHg

   Answer: 736.2 mmHg

   Explanation: Ptotal = Pgas + Pwater. So, Pgas = 760 - 23.8 = 736.2 mmHg. Option B is total pressure, not gas pressure.

10. Question 10

    Q10. The volume of 0.5 mole of an ideal gas at STP is

    • A) 11.2 L
    • B) 22.4 L
    • C) 5.6 L
    • D) 16.8 L

    Answer: 11.2 L

    Explanation: 1 mole occupies 22.4 L at STP, so 0.5 mole occupies 11.2 L. Option C is incorrect due to miscalculation.

11. Question 11

    Q11. A 5 L container holds a mixture of 1 mole of He and 2 moles of Ar at 27°C. What is the total pressure?

    • A) 10 atm
    • B) 12.3 atm
    • C) 14.7 atm
    • D) 20 atm

    Answer: 14.7 atm

    Explanation: Total moles = 3, PV = nRT gives P = 14.7 atm. Option B is incorrect due to wrong calculation.

12. Question 12

    Q12. The ratio of rates of diffusion of two gases is 2:1. If the molar mass of the slower gas is 64 g/mol, what is the molar mass of the faster gas?

    • A) 16 g/mol
    • B) 32 g/mol
    • C) 8 g/mol
    • D) 4 g/mol

    Answer: 16 g/mol

    Explanation: Graham's Law: Rate ratio = √(M2/M1). So, M1 = 16 g/mol. Option B is incorrect molar mass.

13. Question 13

    Q13. At what pressure will 1 mole of an ideal gas occupy 1 L at 273 K?

    • A) 11.2 atm
    • B) 22.4 atm
    • C) 24.0 atm
    • D) 20.0 atm

    Answer: 22.4 atm

    Explanation: Ideal Gas Law: PV = nRT, so P = 22.4 atm. Option C is not the correct pressure.

14. Question 14

    Q14. A gas expands isothermally from 2 L to 4 L. If the initial pressure is 2 atm, what is the final pressure?

    • A) 1 atm
    • B) 2 atm
    • C) 0.5 atm
    • D) 4 atm

    Answer: 1 atm

    Explanation: Boyle's Law: P1V1 = P2V2, so P2 = 1 atm. Option B is initial pressure, not final.

15. Question 15

    Q15. The temperature at which the rms speed of H2 is the same as that of O2 at 300 K is

    • A) 18.75 K
    • B) 37.5 K
    • C) 75 K
    • D) 150 K

    Answer: 18.75 K

    Explanation: urms = √(3RT/M). Setting urms(H2) = urms(O2) gives T(H2) = 18.75 K. Option B is incorrect temperature.

16. Question 16

    Q16. A mixture of 1 mole of CO and 2 moles of CO2 has a total pressure of 3 atm. What is the partial pressure of CO?

    • A) 1 atm
    • B) 2 atm
    • C) 1.5 atm
    • D) 0.5 atm

    Answer: 1 atm

    Explanation: Mole fraction of CO = 1/3, so P(CO) = 1 atm. Option C is incorrect partial pressure.

17. Question 17

    Q17. The density of a gas at 1 atm and 27°C is 1.5 g/L. What is its molar mass?

    • A) 36.9 g/mol
    • B) 44.1 g/mol
    • C) 30.5 g/mol
    • D) 37.4 g/mol

    Answer: 37.4 g/mol

    Explanation: M = dRT/P = 37.4 g/mol. Option A is close but incorrect due to rounding.

18. Question 18

    Q18. A sample of gas occupies 3 L at 1 atm and 300 K. What will be its volume at 2 atm and 400 K?

    • A) 2 L
    • B) 3 L
    • C) 2.25 L
    • D) 4 L

    Answer: 2 L

    Explanation: Combined Gas Law: P1V1/T1 = P2V2/T2 gives V2 = 2 L. Option C is incorrect due to miscalculation.

19. Question 19

    Q19. The partial pressure of oxygen in air at 1 atm is 0.21 atm. What is its mole fraction?

    • A) 0.21
    • B) 0.5
    • C) 0.79
    • D) 1.0

    Answer: 0.21

    Explanation: Mole fraction = partial pressure / total pressure = 0.21. Option B is incorrect mole fraction.

20. Question 20

    Q20. A gas is stored in a cylinder at room temperature. Which gas cannot be liquefied by applying pressure under normal conditions?

    • A) CO2
    • B) NH3
    • C) O2
    • D) SO2

    Answer: O2

    Explanation: O2 has a critical temperature below room temperature. CO2 and NH3 can be liquefied under normal pressure.