Gases and liquids

Written by Dr. Ayesha Nadeem (Chemistry)

MDCAT · Chemistry · 8 questions

Name: ____________________ Date: ____________

At constant pressure, the absolute temperature of a gas is doubled. Its volume:
- A Becomes four times as large
- B Halves
- C Stays the same
- D Doubles
4 L of a gas at 1 atm is compressed to 2 L at constant temperature. What is the new pressure?
- A 4 atm
- B 2 atm
- C 8 atm
- D 0.5 atm
According to Avogadro's law, equal volumes of gases at the same temperature and pressure contain:
- A Equal numbers of atoms
- B Equal densities
- C Equal numbers of molecules
- D Equal masses
Compared with oxygen, hydrogen gas effuses how many times faster under the same conditions? (H = 1, O = 16)
- A 16 times
- B 8 times
- C 2 times
- D 4 times
How many moles of gas occupy 22.4 dm3 at STP?
- A 2
- B 1
- C 22.4
- D 0.5
As the temperature of a liquid rises, its vapour pressure:
- A Stays the same
- B Increases
- C Decreases
- D Becomes zero
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?
- A 2.0 atm
- B 1.0 atm
- C 3.0 atm
- D 0.75 atm
Real gases deviate most from ideal behaviour at:
- A Low temperature and high pressure
- B High temperature and high pressure
- C Low temperature and low pressure
- D High temperature and low pressure

Answer key

1. D, 2. B, 3. C, 4. D, 5. B, 6. B, 7. A, 8. A

Charles's law says that at constant pressure, volume is directly proportional to absolute temperature.
By Boyle's law, P1 x V1 = P2 x V2, so P2 = (1 x 4) / 2 = 2 atm.
Equal volumes contain equal numbers of molecules. The atoms per molecule may differ.
Graham's law: the rate is proportional to 1 / sqrt(molar mass). The ratio is sqrt(32 / 2) = sqrt(16) = 4.
One mole of an ideal gas occupies 22.4 dm3 at STP.
More molecules have enough energy to escape from the surface, so the vapour pressure rises.
Dalton's law says the total pressure is the sum of the partial pressures: 0.5 + 1.5 = 2.0 atm.
At low temperature and high pressure the molecules are close together and attract each other, so the ideal gas assumptions fail.