1.5 Exam Practice
Exam-style practice questions on Ideal Gases
Section B: Data Analysis (Paper 1B Style)
Calculator and Data Booklet permitted. Show all working clearly.
Question 1: Determining Molar Mass from Gas Data Determine
5 marksA student heats a volatile liquid in a conical flask fitted with a pinhole cover. The following data was recorded:
| Measurement | Value |
|---|---|
| Mass of vapour | 0.285 g |
| Temperature of water bath | 99 °C |
| Volume of flask | 149 cm³ |
| Atmospheric pressure | 101.3 kPa |
(a) Convert the temperature to Kelvin and the volume to dm³. [1]
(b) Using the ideal gas equation PV = nRT, calculate the number of moles of vapour. [2]
(c) Hence determine the molar mass of the liquid and suggest its identity. [2]
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(a) T = 99 + 273 = 372 K; V = 149 / 1000 = 0.149 dm³ [1]
(b) n = PV/RT = 101.3 × 0.149/8.314 × 372 [1]
n = 15.094/3092.8 = 4.879 × 10⁻³ mol [1]
(c) M = m/n = 0.285/4.879 × 10⁻³ = 58.4 g mol⁻¹ [1]
This is close to the molar mass of propanone / acetone (C_3H_6O, M = 58.08) [1]
Section C: Structured Questions (Paper 2 Style)
Show all working. State answers with appropriate significant figures and units.
Question 2: Ideal Gas Equation Calculate
4 marksA 5.00 dm³ container holds nitrogen gas at 25 °C and 200 kPa.
(a) Calculate the number of moles of nitrogen in the container. [2]
(b) Calculate the mass of nitrogen in the container. (M of N_2 = 28.02 g mol⁻¹) [1]
(c) If the temperature is increased to 100 °C at constant volume, calculate the new pressure. [1]
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(a) T = 25 + 273 = 298 K
n = PV/RT = 200 × 5.00/8.314 × 298 = 1000/2477.6 = 0.4036 mol [2]
Award [1] for correct substitution with arithmetic error.
(b) Mass = 0.4036 × 28.02 = 11.3 g [1]
(c) P_1/T_1 = P_2/T_2, so P_2 = 200 × 373/298 = 250 kPa [1]
Question 3: Gas Stoichiometry Determine
4 marksMethane burns completely in oxygen: CH_4(g) + 2O_2(g) → CO_2(g) + 2H_2O(g)
(a) Calculate the volume of oxygen (at STP, in dm³) needed to completely burn 100 cm³ of methane. [1]
(b) Calculate the total volume of gaseous products formed (at STP) from 1.00 mol of methane. [2]
(c) State what would happen to the volume of CO_2 produced if the reaction was carried out at 110 °C and 100 kPa instead of STP. [1]
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(a) By Avogadro's law (equal volumes at same T and P), volume ratio = mole ratio. Volume of O_2 = 2 × 100 = 200 cm³ [1]
(b) Moles of CO_2 = 1.00 mol, moles of H_2O(g) = 2.00 mol [1]
Total volume = (1.00 + 2.00) × 22.7 = 68.1 dm³ [1]
(c) The volume would be larger because the temperature is higher (above 0 °C) / gases expand at higher temperatures [1]
Question 4: Real vs Ideal Gases Discuss
4 marks(a) State two assumptions of the ideal gas model. [2]
(b) Explain why ammonia (NH_3) deviates more from ideal behaviour than helium (He) under the same conditions of temperature and pressure. [2]
Show Mark Scheme
(a) Any two of:
- Gas particles have negligible volume compared to the volume of the container [1]
- There are no intermolecular forces between gas particles [1]
- Collisions are perfectly elastic [1]
(b) NH_3 molecules are polar and can form hydrogen bonds, so there are significant intermolecular forces between particles [1]
NH_3 molecules are larger than He atoms, so the volume of the particles is more significant compared to the total volume / the assumption of negligible volume is less valid [1]
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