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S4 Chemistry: Molar Volume and Gas Calculations (Paper 4: Misconception Drill)
S4 Chemistry: Molar Volume and Gas Calculations (Paper 4: Misconception Drill)
Ten items on the commonest errors: mixing up 22.4 and 24, unconverted units, mass treated as moles, volume ratio taken as mass ratio and yields added, with answers.
Direct answers Answers in order: use 24, convert units to 0.010 mol, divide the mass by 32 first, volume 12 dm3, mass ratio 4 to 32, oxygen runs out with hydrogen left, multiply the yield to get 3.0 dm3, multiply by 24 to get 30.0, molar volume varies with conditions, water is a liquid so it does not apply.
01 Paper details (check answers afterwards)
| Item | Detail |
|---|---|
| Questions | 10 |
| Suggested time | 20 |
| Difficulty mix | Foundation 3 · Intermediate 3 · Challenge 3 · Error redo 1 |
| Chapter | Chapter: S4 Chemistry: Molar Volume and Gas Calculations |
| Level | S4 |
02 Questions (10 items)
- FoundationA question states room temperature and pressure, but a student uses 22.4 dm3/mol. What is wrong?
- FoundationDividing 240 cm3 by 24 directly gives 10 mol. What is wrong?
- FoundationFor 48 g of oxygen with O2 equal to 32, a student divides 48 by 24 and gets 2 mol. What is wrong?
- IntermediateFor 0.50 mol of gas a student answers that the volume is 0.50. What is wrong?
- IntermediateThe volume ratio of hydrogen to oxygen in a reaction is 2 to 1, and a student concludes the mass ratio is also 2 to 1. What is wrong?
- Intermediate9.0 dm3 of hydrogen is mixed with 9.0 dm3 of oxygen and ignited, and a student adds them to say 18 dm3 is used. What is wrong?
- ChallengeThe theoretical volume is 4.0 dm3 and the yield is 75.0 percent, but a student answers 4.75 dm3. What is wrong?
- ChallengeA gas has a density of 1.25 g/dm3 and a student multiplies by 22.4 to find the relative molecular mass. What is wrong?
- ChallengeA student says the volume of a gas is fixed, so 1 mol is always 24 dm3. What is wrong?
- Error redoA student uses the molar volume to say 1 mol of water occupies 24 dm3 at room temperature and pressure. What is wrong?
03 Answers and one-line notes
| # | Answer | One-line note |
|---|---|---|
| 1 | The student used the s.t.p. molar volume, while room conditions require 24 dm3/mol. | The value 22.4 dm3 belongs to 0 degrees Celsius and 1 atm, so it cannot be carried over to different conditions. |
| 2 | The units were not converted. 240 cm3 is 0.240 dm3, and dividing by 24 gives 0.010 mol. | There are 1 000 cm3 in a dm3 and the molar volume is quoted per dm3, so the units must be matched first. |
| 3 | The mass was treated as moles. 48 divided by 32 is 1.5 mol and the volume is 1.5 times 24, that is 36 dm3. | The mass must be divided by the relative molecular mass before dividing by 24, otherwise the quantity is still in grams. |
| 4 | Only the moles were given. The volume is 0.50 times 24, that is 12 dm3, and the unit must be stated. | Moles multiplied by the molar volume give the volume, and the answer should be checked for the unit dm3. |
| 5 | The volume ratio is the mole ratio, and the mass ratio needs each relative molecular mass, giving 4 to 32 for hydrogen and oxygen. | Gas volume reflects only the number of molecules, so different molecular masses give different masses. |
| 6 | The mole ratio was not compared. The equation needs two volumes of hydrogen to one of oxygen, so 0.375 mol of oxygen can only use 0.75 mol of hydrogen and hydrogen is left over. | The limiting reagent is found by dividing moles by coefficients, not by adding the volumes. |
| 7 | The percentage was added instead of multiplied. 4.0 times 0.750 gives 3.0 dm3. | A yield is a fraction of the theoretical amount, so multiplying can never give more than the theoretical value. |
| 8 | The wrong molar volume was used. Room conditions need 24, giving 30.0, while 22.4 giving 28.0 applies only at s.t.p. | The key in density questions is to check the stated temperature and pressure before choosing between 24 and 22.4. |
| 9 | The molar volume changes with temperature and pressure, and 24 dm3 is only the room condition value, so the conditions must be stated. | Heating a gas expands it and compressing it shrinks it, so the same moles occupy different volumes under different conditions. |
| 10 | Wrong. Water is a liquid at room conditions and the molar volume applies to gases only; 1 mol of water is about 18 cm3. | The value 24 dm3/mol assumes a gas, while liquids and solids need their density to find the volume. |
04 What to do after marking
- After marking, redo only the wrong items and write down which step went wrong (formula, unit, or misread number).
- Repeat the same item two days later; move to the next paper only when both attempts are correct.
- Parents only need the answer and note columns; there is no need to explain every item.
Turn every wrong item today into one sentence: I thought this, but actually that.Tutor Registration →
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