S1.5 Ideal gases
Practice exam-style IB Chemistry questions for Ideal gases, aligned with the syllabus and grouped by topic.
Question 1
What assumption is made about particles in an ideal gas?
A.
They lose kinetic energy during collisions with the walls.
B.
They attract each other strongly at all temperatures.
C.
They have negligible volume compared with the container volume.
D.
They move in circular paths between collisions.
Question 2
A fixed amount of ideal gas is kept at constant temperature. The volume is doubled. What happens to the pressure?
A.
It becomes four times smaller.
B.
It is doubled.
C.
It is halved.
D.
It remains unchanged.
Question 3
What volume is occupied by 0.250 mol of an ideal gas at STP? Use the molar volume at STP as 22.7 dm³ mol⁻¹.
A.
90.8 dm³
B.
22.7 dm³
C.
5.68 dm³
D.
45.4 dm³
Question 4
A student uses PV = nRT with R = 8.31 J K⁻¹ mol⁻¹. What set of units is consistent with this value of R?
A.
P in kPa, V in dm³, T in °C
B.
P in kPa, V in m³, T in °C
C.
P in Pa, V in m³, T in K
D.
P in Pa, V in dm³, T in K
Question 5
State one assumption about the volume of particles in the ideal gas model.
[1]
State what is meant by an elastic collision.
[1]
Question 6
State the temperature and pressure conditions under which a real gas is most likely to deviate from ideal behaviour.
[1]
Outline why low temperature causes greater deviation.
[1]
Question 7
Under what conditions does a real gas show the greatest deviation from ideal behaviour?
A.
High temperature and low pressure
B.
High temperature and high pressure
C.
Low temperature and low pressure
D.
Low temperature and high pressure
Question 8
A fixed amount of ideal gas has P = 120 kPa, V = 2.00 dm³ and T = 300 K. The gas is heated at constant volume to 450 K. What is the final pressure?
A.
80.0 kPa
B.
270 kPa
C.
180 kPa
D.
120 kPa
Question 9
For a fixed amount of ideal gas at constant pressure, what graph is expected when volume is plotted against absolute temperature?
A.
A straight line through the origin with positive gradient
B.
A horizontal straight line
C.
A curve decreasing towards zero
D.
A straight line with negative gradient
Question 10
A gas sample is collected over water. The total pressure is 101.3 kPa and the water vapour pressure is 3.2 kPa. What pressure should be used for the dry gas in PV = nRT?
A.
3.2 kPa
B.
104.5 kPa
C.
98.1 kPa
D.
101.3 kPa
Question 11
A gas exerts pressure on the walls of its container. What is the microscopic cause of this pressure?
A.
Particles attract the walls by intermolecular forces.
B.
Particles lose kinetic energy to the walls continuously.
C.
Particles collide with the walls and transfer momentum.
D.
Particles occupy most of the container volume.
Question 12
For a fixed mass of ideal gas, which plotted relationship gives a straight line through the origin when temperature is constant?
A.
P against V
B.
V against 1/P²
C.
PV against V
D.
P against 1/V
Question 13
A sample of oxygen has a volume of 2.50 dm³ at 100 kPa and 298 K.
Convert the volume to m³.
[1]
Calculate the amount, in mol, of oxygen. Use R = 8.31 J K⁻¹ mol⁻¹.
[1]
Question 14
A sealed rigid container contains a fixed amount of ideal gas.
State the relationship between pressure and absolute temperature.
[1]
Explain this relationship using the particle model.
[1]
Question 15
A gas in a syringe has a volume of 60.0 cm³ at 300 K and 100 kPa. The pressure is increased to 150 kPa at constant temperature.
State the gas law relationship used.
[1]
Calculate the final volume.
[1]
Question 16
A student investigates the relationship between pressure and volume for a fixed amount of gas at constant temperature.
Describe the trend shown in the graph.
[1]
Identify the variable that should be plotted on the x-axis to obtain a straight line through the origin.
[1]
Explain, using the particle model, why the pressure increases when the volume decreases.
[1]
State one controlled variable in this investigation.
[1]
Question 17
A table gives measurements for a fixed amount of gas heated in a rigid container.
| Temperature / K | Pressure / kPa |
|---|---|
| 280 | 92.0 |
| 300 | 98.7 |
| 320 | 105.9 |
| 340 | 112.1 |
| 360 | 119.2 |
| 380 | 125.4 |
Identify the independent variable.
[1]
State the relationship between pressure and temperature shown by the data.
[1]
Use the data to predict the pressure at a temperature within the measured range.
[1]
Explain why temperature must be expressed in kelvin for this relationship.
[1]
Question 18
A gas is expected to deviate more from ideal behaviour than helium under the same conditions. What property best explains this?
A.
A smaller molar mass and no intermolecular attractions
B.
A larger molar mass and stronger intermolecular attractions
C.
A lower density caused only by a larger container
D.
A higher average kinetic energy at the same temperature
Question 19
A sealed flexible balloon contains an ideal gas at 300 K and 100 kPa. It is taken to a place where the pressure is 80 kPa and its temperature is 240 K. What happens to its volume, assuming the amount of gas is unchanged?
A.
It increases to 1.25 times the original volume.
B.
It decreases to 0.64 of the original volume.
C.
It remains the same.
D.
It decreases to 0.80 of the original volume.
Question 20
A student predicts the pressure of ammonia gas using the ideal gas equation at low temperature. The measured pressure is lower than predicted. What is the best explanation?
A.
Elastic collisions increase the pressure above the prediction.
B.
Attractions between molecules reduce the force of wall collisions.
C.
The gas constant becomes smaller at low temperature.
D.
The molar mass of ammonia changes at low temperature.
Question 21
A 0.100 g sample of a gas occupies 83.1 cm³ at 100 kPa and 300 K. What is its approximate molar mass?
A.
3.00 g mol⁻¹
B.
3000 g mol⁻¹
C.
300 g mol⁻¹
D.
30.0 g mol⁻¹
Question 22
Methane, chloromethane and hydrogen chloride are compared at the same temperature and pressure. Which gas is likely to show the largest deviation from ideal behaviour?
A.
All three, because equal volumes contain equal numbers of particles
B.
Chloromethane, because it is polar and has a larger electron cloud
C.
Methane, because it is non-polar and small
D.
Hydrogen chloride, because it has the lowest boiling point
Question 23
A molar mass experiment gives a value lower than the accepted value. The gas was collected over water but the water vapour pressure was not subtracted. What is the effect of this error?
A.
The temperature is underestimated, so the amount is too low.
B.
The calculated amount of gas is too low, so the molar mass is too high.
C.
The calculated amount of gas is too high, so the molar mass is too low.
D.
The calculated volume is too high, so the molar mass is too high.
Question 24
At STP, the molar volume of an ideal gas is 22.7 dm³ mol⁻¹. A reaction produces 454 cm³ of carbon dioxide at STP.
Convert 454 cm³ to dm³.
[1]
Determine the amount of carbon dioxide produced.
[1]
State one assumption made when using molar volume in this calculation.
[1]
Question 25
Nitrogen and sulfur dioxide are compared at the same temperature and pressure.
State which gas is expected to behave more ideally.
[1]
Explain your choice.
[1]
Question 26
A gas sample is collected over water. The total pressure is 99.8 kPa, the water vapour pressure is 2.6 kPa, the volume is 125 cm³ and the temperature is 298 K.
Determine the pressure of the dry gas in Pa.
[1]
Calculate the amount of dry gas, in mol.
[1]
Question 27
A molar mass experiment for a gas gives inconsistent results between trials.
State one random error that could affect the measured gas volume.
[1]
Suggest two improvements to reduce uncertainty or improve reliability.
[1]
Question 28
A sample of carbon dioxide is compressed to a high pressure at room temperature.
State one ideal gas assumption that becomes less valid.
[1]
Explain why high pressure increases deviation from ideal behaviour.
[1]
Question 29
Two gases at the same temperature are helium and xenon.
Compare their average kinetic energies.
[1]
Compare their average molecular speeds.
[1]
Explain the answer to (b).
[1]
Question 30
A sample of gas is collected in a gas syringe and the measurements are shown.
| Pressure / kPa | Volume / cm³ | Temperature / °C | Mass of gas / g |
|---|---|---|---|
| 101 | 250 | 25.0 | 0.450 |
Convert the measured volume from cm³ to m³.
[1]
Calculate the amount of gas using PV = nRT.
[1]
The mass of gas collected is given in the table. Calculate the molar mass.
[1]
State one assumption made in the calculation.
[1]
Question 31
A student presents two representations of the relationship between volume and absolute temperature at constant pressure: a sketch graph and a plotted graph of measured data.
Identify the ideal relationship shown by the sketch graph.
[1]
State one feature of the plotted data that cannot be shown by a simple sketch.
[1]
Suggest one reason why a measured point may not lie exactly on the ideal line.
[1]
Evaluate which representation is more useful for judging the reliability of the experiment.
[1]
Question 32
The graph compares the compressibility of a real gas with ideal gas behaviour at two temperatures.
Identify the temperature at which the gas behaves more ideally over the pressure range shown.
[1]
Describe how the real gas curve at low temperature differs from the ideal line at high pressure.
[1]
Explain one reason for the deviation at high pressure.
[1]
State a condition, other than pressure, that would reduce deviation from ideal behaviour.
[1]
Question 33
A fixed amount of gas occupies 1.20 dm³ at 290 K and 95.0 kPa. It is compressed to 0.800 dm³ and heated to 350 K.
State why the combined gas law can be used.
[1]
Calculate the final pressure, in kPa.
[1]
Question 34
In an experiment, 0.184 g of a volatile liquid is vaporized. The vapour occupies 76.0 cm³ at 373 K and 101 kPa.
Calculate the amount of vapour.
[1]
Determine the molar mass of the volatile liquid.
[1]
Question 35
A graph of pressure against reciprocal volume for a fixed amount of gas at constant temperature is a straight line through the origin.
State the relationship shown by the graph.
[1]
Explain why plotting pressure against reciprocal volume is useful.
[1]
State one limitation of using a sketch graph instead of plotted data.
[1]
Question 36
A student calculates the molar mass of a gas from the mass lost by a gas generator and the gas volume collected in a syringe.
State the equation used to calculate the amount of gas from the measured pressure, volume and temperature.
[1]
Suggest how each error would affect the calculated molar mass: gas leaks before entering the syringe; temperature is recorded lower than the actual gas temperature.
[1]
Question 37
A sealed rigid vessel contains 0.250 mol of ideal gas at 27.0 °C. The pressure is 208 kPa.
Convert the temperature to K.
[1]
Calculate the volume of the vessel in dm³.
[1]
Question 38
A gas collected over water is used to determine molar mass. The table includes total pressure, water vapour pressure, gas volume, temperature and mass collected.
| Total pressure / kPa | Water vapour pressure / kPa | Gas volume / cm³ | Temperature / °C | Mass collected / g |
|---|---|---|---|---|
| 101.3 | 3.17 | 250.0 | 25.0 | 0.436 |
Determine the pressure of the dry gas.
[1]
Calculate the amount of dry gas.
[1]
Calculate the molar mass of the gas.
[1]
Suggest how failing to correct for water vapour would affect the calculated molar mass.
[1]
Question 39
The table compares four gases at the same temperature and pressure, showing polarity, relative molecular size and observed deviation from ideal behaviour.
| Gas | Polarity | Relative size | Deviation from ideal |
|---|---|---|---|
| Helium (He) | Non-polar | Very small | Very low |
| Methane (CH₄) | Non-polar | Small | Low |
| Ammonia (NH₃) | Polar | Small | Moderate |
| Sulfur dioxide (SO₂) | Polar | Large | High |
Identify the gas with the smallest deviation.
[1]
Describe the relationship between molecular size and deviation shown by the table.
[1]
Explain why a polar gas may deviate more than a non-polar gas of similar size.
[1]
Suggest one limitation of using only this table to predict ideality for other conditions.
[1]
Question 40
A gas syringe experiment records pressure and volume before and after a temperature change for the same gas sample.
| P₁ / kPa | V₁ / cm³ | T₁ / K | P₂ / kPa | T₂ / K | Measured V₂ / cm³ |
|---|---|---|---|---|---|
| 100.0 | 50.0 | 298 | 120.0 | 333 | 45.8 |
Use the initial data to calculate P₁V₁/T₁.
[1]
Use the final pressure and temperature to predict the final volume.
[1]
Suggest one experimental reason for a difference between predicted and measured final volume.
[1]
Question 41
Outline two assumptions of the ideal gas model.
[1]
Explain how the ideal gas model accounts for pressure and for the effect of increasing temperature at constant volume.
[1]
Question 42
State the conditions under which real gases most closely approximate ideal behaviour, and the conditions under which they deviate most.
[1]
Discuss why real gases deviate from ideal behaviour, referring to particle volume, intermolecular forces and molecular structure.
[1]
Question 43
State Avogadro’s law and the molar volume of an ideal gas at STP used in IB calculations.
[1]
Compare the information obtained from a sketch graph and an accurately plotted graph when investigating gas relationships.
[1]
Question 44
A spreadsheet output is shown for an experiment on a fixed amount of gas at constant temperature. It includes volume, pressure and calculated reciprocal volume.
Identify the graph that should be linear if the gas behaves ideally.
[1]
Use the gradient of the linear graph to determine the value of PV for the sample.
[1]
Use PV = nRT to determine the amount of gas from the gradient.
[1]
Suggest why the intercept of the best-fit line may not be exactly zero.
[1]
Question 45
A graph shows the ratio of measured molar volume to ideal molar volume for two gases as pressure increases at constant temperature.
State the ratio expected for ideal behaviour.
[1]
Identify which gas deviates more from ideal behaviour at high pressure.
[1]
Explain why the deviation may be positive at very high pressure.
[1]
Suggest why lowering the temperature would change the curves.
[1]
Question 46
A 0.320 g sample of gas occupies 250 cm³ at 298 K and 100 kPa.
Convert the pressure and volume into SI units.
[1]
Calculate the molar mass of the gas and explain why temperature must be converted to kelvin in gas calculations.
[1]
Question 47
A student determines the molar mass of a gas by collecting it over water.
Outline the calculations needed to obtain the amount of dry gas from the experimental measurements.
[1]
Evaluate the method by discussing corrections, assumptions and possible sources of systematic error affecting the calculated molar mass.
[1]
Question 48
Outline why ammonia is expected to deviate more from ideal behaviour than neon under the same conditions.
[1]
Compare and contrast the effects of low temperature and high pressure on real gas behaviour.
[1]
Question 49
A class uses a pressure sensor and a variable-volume syringe to investigate a fixed amount of gas at constant temperature.
Describe how the data should be processed to test the ideal relationship between pressure and volume.
[1]
Evaluate the quality of the investigation by considering control variables, graphical evidence and limitations of the ideal model.
[1]
Question 50
A sealed gas cylinder is moved from a cool room to direct sunlight. The cylinder volume is constant and no gas escapes.
Use the combined gas law to derive the relationship between pressure and temperature for this situation.
[1]
Discuss the prediction of the ideal gas model for the cylinder and the limitations of applying the model to a real gas in this context.
[1]
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