Cambridge International AS & A Level · 15 · structured
What an ideal gas model assumes
What an ideal gas model assumes · Original GioPhysics question with a detailed, mark-by-mark answer guide.
- Demand
- demanding
- Marks
- 8
- Topics
- 3
- Answer
- Complete
A sealed dilute gas is described using amount of substance n and a kinetic model. Answer without treating all molecules as having one speed.
- (a)
Name the SI base quantity represented by n and its base unit.
1 mark - (b)
State the defining p,V,T relationship for a fixed amount of ideal gas, identify the temperature scale and predict p₂ when p₁=1.00×10⁵ Pa, T₁=300 K and T₂=450 K at unchanged volume.
3 marks - (c)
State the basic particle-model assumptions about molecular size, forces, motion and collisions.
4 marks
Ready to self-mark?Reveal the detailed answer guide
(a)
Name the SI base quantity represented by n and its base unit.
Identify what the question is testing, organise the response into distinct mark-earning points, and make every conclusion traceable to a physical principle or to the evidence supplied.
- 1
n is amount of substance, an SI base quantity measured in mol.
(b)
State the defining p,V,T relationship for a fixed amount of ideal gas, identify the temperature scale and predict p₂ when p₁=1.00×10⁵ Pa, T₁=300 K and T₂=450 K at unchanged volume.
List the given quantities with units, identify the required quantity, write the governing relationship before substituting, and keep extra digits until the final line so rounding does not distort the result.
- 1
For a fixed amount of ideal gas, pV/T is constant.
- 2
T must be thermodynamic temperature in kelvin.
- 3
With V fixed, p₂=p₁T₂/T₁=1.00×10⁵×450/300=1.50×10⁵ Pa.
(c)
State the basic particle-model assumptions about molecular size, forces, motion and collisions.
Identify what the question is testing, organise the response into distinct mark-earning points, and make every conclusion traceable to a physical principle or to the evidence supplied.
- 1
Molecular volume is negligible compared with the occupied gas volume.
- 2
Intermolecular forces are neglected except during collisions.
- 3
A very large number of molecules move randomly with no preferred direction; between collisions they travel at constant velocity.
- 4
Collisions with walls and other molecules are brief and perfectly elastic; no same-speed assumption is required.
Complete marking guidance
Use these worked steps alongside the part-by-part answers. They explain the method and mark allocation; they do not add extra marks.
- (a) n is amount of substance, an SI base quantity measured in mol.
- (b) For a fixed amount of ideal gas, pV/T is constant.
- (b) T must be thermodynamic temperature in kelvin.
- (b) With V fixed, p₂=p₁T₂/T₁=1.00×10⁵×450/300=1.50×10⁵ Pa.
- (c) Molecular volume is negligible compared with the occupied gas volume.
- (c) Intermolecular forces are neglected except during collisions.
- (c) A very large number of molecules move randomly with no preferred direction; between collisions they travel at constant velocity.
- (c) Collisions with walls and other molecules are brief and perfectly elastic; no same-speed assumption is required.
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