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Gases, rates and equilibrium
7 ECAT Chemistry questions with answers and explanations.
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What volume does 1 mole of an ideal gas occupy at 273 K and 1 atm?
- 1 24 L
- 2 11.2 L
- 3 22.4 L
- 4 1 L
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Correct answer: 22.4 L
The molar volume of an ideal gas at 273 K and 1 atm is 22.4 litres.
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According to Boyle's law, if the pressure of a gas is doubled at constant temperature, its volume:
- 1 Doubles
- 2 Halves
- 3 Stays the same
- 4 Becomes four times
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Correct answer: Halves
At constant temperature, pressure times volume is constant, so doubling the pressure halves the volume.
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For a first order reaction, the half-life:
- 1 Depends on the starting concentration
- 2 Is independent of the starting concentration
- 3 Doubles each half-life
- 4 Is zero
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Correct answer: Is independent of the starting concentration
The half-life of a first order reaction is 0.693 / k, which does not depend on concentration.
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Which is the expression for Kc of the reaction N2O4 <-> 2NO2?
- 1 [NO2] / [N2O4]
- 2 [N2O4] / [NO2]^2
- 3 [NO2]^2 / [N2O4]
- 4 [NO2]^2 x [N2O4]
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Correct answer: [NO2]^2 / [N2O4]
Kc is products over reactants, with each concentration raised to the power of its coefficient.
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In a buffer solution, pH = pKa when:
- 1 [A-] is twice [HA]
- 2 [A-] equals [HA]
- 3 [HA] is zero
- 4 The solution is neutral
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Correct answer: [A-] equals [HA]
By the Henderson-Hasselbalch equation, pH = pKa + log([A-] / [HA]), and the log term is zero when the two are equal.
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Hess's law says that the enthalpy change of a reaction:
- 1 Depends on the path taken
- 2 Is the same whether it takes place in one step or several
- 3 Is always negative
- 4 Depends only on the catalyst
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Correct answer: Is the same whether it takes place in one step or several
Enthalpy is a state function, so only the starting and final states matter.
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Which solution has the highest boiling point at the same molality?
- 1 Glucose
- 2 Urea
- 3 Sodium chloride
- 4 Calcium chloride
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Correct answer: Calcium chloride
Boiling point elevation depends on the number of particles. CaCl2 gives three ions, NaCl gives two, and glucose and urea do not ionise.