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Gases and liquids
8 MDCAT Chemistry questions with answers and explanations.
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At constant pressure, the absolute temperature of a gas is doubled. Its volume:
- 1 Halves
- 2 Doubles
- 3 Stays the same
- 4 Becomes four times as large
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Correct answer: Doubles
Charles's law says that at constant pressure, volume is directly proportional to absolute temperature.
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4 L of a gas at 1 atm is compressed to 2 L at constant temperature. What is the new pressure?
- 1 0.5 atm
- 2 2 atm
- 3 4 atm
- 4 8 atm
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Correct answer: 2 atm
By Boyle's law, P1 x V1 = P2 x V2, so P2 = (1 x 4) / 2 = 2 atm.
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According to Avogadro's law, equal volumes of gases at the same temperature and pressure contain:
- 1 Equal masses
- 2 Equal numbers of molecules
- 3 Equal densities
- 4 Equal numbers of atoms
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Correct answer: Equal numbers of molecules
Equal volumes contain equal numbers of molecules. The atoms per molecule may differ.
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Compared with oxygen, hydrogen gas effuses how many times faster under the same conditions? (H = 1, O = 16)
- 1 2 times
- 2 4 times
- 3 8 times
- 4 16 times
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Correct answer: 4 times
Graham's law: the rate is proportional to 1 / sqrt(molar mass). The ratio is sqrt(32 / 2) = sqrt(16) = 4.
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How many moles of gas occupy 22.4 dm3 at STP?
- 1 0.5
- 2 1
- 3 2
- 4 22.4
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As the temperature of a liquid rises, its vapour pressure:
- 1 Decreases
- 2 Increases
- 3 Stays the same
- 4 Becomes zero
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Correct answer: Increases
More molecules have enough energy to escape from the surface, so the vapour pressure rises.
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A mixture holds gas A at a partial pressure of 0.5 atm and gas B at 1.5 atm. What is the total pressure?
- 1 1.0 atm
- 2 0.75 atm
- 3 2.0 atm
- 4 3.0 atm
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Correct answer: 2.0 atm
Dalton's law says the total pressure is the sum of the partial pressures: 0.5 + 1.5 = 2.0 atm.
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Real gases deviate most from ideal behaviour at:
- 1 High temperature and low pressure
- 2 Low temperature and high pressure
- 3 High temperature and high pressure
- 4 Low temperature and low pressure
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Correct answer: Low temperature and high pressure
At low temperature and high pressure the molecules are close together and attract each other, so the ideal gas assumptions fail.