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Gases

23 free MDCAT Chemistry MCQs on Gases (PM&DC syllabus), each with the answer, an explanation and the textbook line it comes from. Tap to check, no sign-up.

MDCAT (Pakistan)ChemistryPre-medical (Grade 12)23 questions

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  1. Question 1

    The term plasma for an ionized gas was first used in 1928 by:

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    Answer: C) Irving Langmuir

    Langmuir applied the term to ionized gas; plasma is the fourth state of matter. Source: Chemistry 11 KPK (taleem360.com), p. 146.

  2. Question 2

    In the NH3/HCl diffusion tube, the white NH4Cl ring forms 60 cm from the NH3 end and 40 cm from the HCl end. The ratio of rates of diffusion of NH3 to HCl is:

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    Answer: D) 1.5

    Distances in the same time give rNH3/rHCl = 60/40 = 1.5, matching the square root of 36.5/17. Source: Chemistry 11 KPK (taleem360.com), p. 138.

  3. Question 3

    The ratio of the volume to the absolute temperature of a fixed mass of gas remains constant at constant pressure. This is:

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    Answer: B) Charles's law

    Charles's law: V/T = constant at constant P. Source: Chemistry 11 KPK (taleem360.com), p. 119.

  4. Question 4

    Extrapolating the volume-temperature graph of a gas, its volume would become zero at:

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    Answer: D) -273 C

    This lowest possible temperature is absolute zero, 0 K. Source: Chemistry 11 KPK (taleem360.com), p. 120.

  5. Question 5

    The total pressure of a mixture of non-reacting gases equals the sum of their partial pressures. This is:

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    Answer: B) Dalton's law

    P(total) = pA + pB + pC + ... Source: Chemistry 11 KPK (taleem360.com), p. 136.

  6. Question 6

    Under the same conditions, the rates of diffusion of gases are inversely proportional to the square roots of their densities. This is:

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    Answer: C) Graham's law

    r1/r2 = square root of (d2/d1) = square root of (M2/M1). Source: Chemistry 11 KPK (taleem360.com), p. 138.

  7. Question 7

    When a compressed gas expands suddenly into a region of low pressure, it cools. This is the:

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    Answer: D) Joule-Thomson effect

    Molecules use their own energy to overcome attractions as they separate; Linde's liquefaction is based on it. Source: Chemistry 11 KPK (taleem360.com), p. 144.

  8. Question 8

    One pascal, the SI unit of pressure, is equal to one:

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    Answer: B) Newton per square metre

    Pressure = force / area, so 1 Pa = 1 N acting on 1 m2 = 1 N m-2. Source: Chemistry 11 KPK (taleem360.com), p. 116.

  9. Question 9

    Under the same conditions, how many times faster does H2 (M = 2) diffuse than CO2 (M = 44)?

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    Answer: A) 4.7

    rH2/rCO2 = square root of (44/2) = square root of 22 = 4.7. Source: Chemistry 11 KPK (taleem360.com), p. 141.

  10. Question 10

    A gas pressure of 49 torr is equal to about:

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    Answer: B) 0.0645 atm

    49 / 760 = 0.0645 atm. Source: Chemistry 11 KPK (taleem360.com), p. 116.

  11. Question 11

    At the same temperature, molecules of different gases have the same:

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    Answer: B) Average kinetic energy

    Kinetic energy depends only on temperature; lighter molecules therefore move faster. Source: Chemistry 11 KPK (taleem360.com), p. 142.

  12. Question 12

    A gas occupies 2 dm3 at 1 atm. At the same temperature, its volume at 4 atm will be:

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    Answer: C) 0.5 dm3

    P1V1 = P2V2: V2 = 1 x 2 / 4 = 0.5 dm3. Source: Chemistry 11 KPK (taleem360.com), p. 118.

  13. Question 13

    At constant pressure, if a gas at 0 C is heated to 273 C, its volume:

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    Answer: A) Doubles

    Volume grows by 1/273 per degree from 0 C, so +273 degrees adds 273/273: the volume doubles (273 K to 546 K). Source: Chemistry 11 KPK (taleem360.com), p. 119.

  14. Question 14

    A temperature of 25 C on the Kelvin scale is:

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    Answer: B) 298 K

    K = C + 273 = 25 + 273 = 298 K. Source: Chemistry 11 KPK (taleem360.com), p. 120.

  15. Question 15

    What is the mass of 1 dm3 of NH3 (17 g/mol) at 1000 mm Hg and 30 C, treating it as ideal?

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    Answer: A) 0.90 g

    m = PVM/RT = (1000/760) x 1 x 17 / (0.0821 x 303) = 0.90 g. Source: Chemistry 11 KPK (taleem360.com), p. 126.

  16. Question 16

    The density of SO2 (64 g/mol) at 20 C and 750 mm Hg is about:

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    Answer: B) 2.63 g/dm3

    d = PM/RT = (750/760) x 64 / (0.0821 x 293) = 2.63 g/dm3. Source: Chemistry 11 KPK (taleem360.com), p. 128.

  17. Question 17

    At very high pressure, real gases deviate from ideal behaviour mainly because:

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    Answer: D) Molecular volume is no longer negligible

    Squeezed together, the molecules' own volume becomes a significant part of the total, and attractions grow. Source: Chemistry 11 KPK (taleem360.com), p. 132.

  18. Question 18

    In the van der Waals equation, the constant b corrects for the:

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    Answer: A) Excluded volume of molecules

    b is the excluded volume (about four times the actual molecular volume); a corrects for attractions. Source: Chemistry 11 KPK (taleem360.com), p. 134.

  19. Question 19

    In the van der Waals equation, the constant a accounts for:

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    Answer: B) Intermolecular attraction

    a is the coefficient of attraction; it corrects the pressure. Source: Chemistry 11 KPK (taleem360.com), p. 135.

  20. Question 20

    A gas mixture at a total pressure of 50 atm contains 9.6% NH3 (by moles). The partial pressure of NH3 is:

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    Answer: C) 4.8 atm

    pNH3 = 9.6/100 x 50 = 4.8 atm. Source: Chemistry 11 KPK (taleem360.com), p. 136.

  21. Question 21

    The escape of gas molecules one by one through a hole of molecular size, as in a punctured tyre, is called:

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    Answer: D) Effusion

    A puncture: effusion. A burst: diffusion (sudden spreading and mixing). Source: Chemistry 11 KPK (taleem360.com), p. 137.

  22. Question 22

    The temperature above which a gas cannot be liquefied, however great the pressure, is its:

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    Answer: A) Critical temperature

    Above Tc attractions cannot hold the molecules together; the minimum pressure needed at Tc is the critical pressure. Source: Chemistry 11 KPK (taleem360.com), p. 143.

  23. Question 23

    Plasma, the fourth state of matter, is a mixture of:

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    Answer: B) Ions, electrons and neutral particles

    Heating vapours strongly frees electrons; plasma conducts electricity and responds to magnetic fields. Source: Chemistry 11 KPK (taleem360.com), p. 147.

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