Gaseous Volume Calculations (OCR A-Level Chemistry): Worked Exam Question & Exam Technique
- Sep 17, 2019
- 5 min read
Updated: Jul 25
By Paul Morgan
Many OCR A-Level Chemistry students lose straightforward marks on gaseous volume questions—not because the calculations are difficult, but because they overcomplicate them.
A common mistake is converting gas volumes into moles before doing any calculations. In many OCR exam questions, this isn't necessary at all.
If every gas is measured at the same temperature and pressure, the balanced chemical equation already tells you everything you need to know.
In this guide you'll learn:
How gaseous volume calculations work
How to identify the limiting reagent quickly
A fully worked OCR-style exam question
The most common mistakes students make
Practical exam tips that save time and marks
If you'd prefer to watch the explanation first, the video below walks through the worked example step by step.
Why Gaseous Volume Questions Are Easier Than They Look
Many students see gas volumes measured in cm³ and immediately think they need to convert everything into moles.
In many OCR A-Level Chemistry questions, that's unnecessary.
When gases are measured under the same temperature and pressure, equal volumes contain equal numbers of particles. This means the coefficients in the balanced equation can be used directly as gas volume ratios.
Instead of converting to moles first, you can compare the gas volumes immediately using the balanced equation.
Recognising this saves valuable time in exams and reduces the chance of making calculation errors.
The Golden Rule
Whenever an exam question tells you that gases are measured at the same temperature and pressure, compare their volumes directly using the balanced equation.
Don't create extra work by converting cm³ into moles unless the question gives different conditions or specifically requires it.
Step-by-Step Worked OCR Exam Question
Question
30 cm³ of xenon reacts with 20 cm³ of fluorine to form xenon fluoride.
What volume of gas remains at the end of the reaction?
Step 1 – Write the Balanced Equation
Xe + F₂ → XeF₂
The balanced equation tells us the gases react in the ratio:
1 : 1 : 1
One volume of xenon reacts with one volume of fluorine to produce one volume of xenon difluoride.
This ratio is the key to the whole question.
Step 2 – Identify the Limiting Reagent
Start by asking:
"Do I have enough fluorine for all of the xenon?"
30 cm³ Xe would require: 30 cm³ F₂
Only 20 cm³ F₂ is available.
Therefore:
Fluorine is the limiting reagent.
This means all of the fluorine reacts and none remains at the end of the reaction.
Step 3 – Calculate How Much Xenon Reacts
The ratio is 1 : 1.
20 cm³ F₂ reacts with:20 cm³ Xe
Step 4 – Calculate the Excess Xenon
Initial xenon:
30 cm³
Reacted:
20 cm³
Remaining:
10 cm³ Xe
Step 5 – Calculate the Product Formed
Again using the 1 : 1 ratio:
20 cm³ XeF₂ forms.
Step 6 – Calculate the Total Gas Remaining
Remaining xenon:
10 cm³
Product formed:
20 cm³ XeF₂
Total gas volume present:
30 cm³
Final Answer
30 cm³ of gas remains.
This consists of:
10 cm³ xenon
20 cm³ xenon difluoride
If the question only asks which reactant remains, the answer is:
10 cm³ of xenon.
Always read the wording carefully before writing your final answer.
Why This Method Works
Students often memorise the method without understanding why it works.
The reason is Avogadro's Law.
At the same temperature and pressure:
equal volumes of gases contain equal numbers of particles.
This means gas volumes behave exactly like mole ratios.
So if the balanced equation shows:
1 molecule reacts with 1 molecule,
then:
1 cm³ reacts with 1 cm³,
10 cm³ reacts with 10 cm³,
100 cm³ reacts with 100 cm³.
The ratio never changes.
Understanding this principle makes gaseous volume questions much easier to recognise in OCR exams.
Common OCR Exam Mistakes
1. Converting Everything Into Moles
Many students automatically start calculating moles.
If every gas is measured under the same conditions, this usually wastes time.
Use the volume ratios directly.
2. Forgetting the Balanced Equation
Never compare the numbers in the question before balancing the equation.
The coefficients determine the reacting volumes.
3. Ignoring the Limiting Reagent
Always identify which reactant runs out first.
Only then calculate any excess remaining.
Students who skip this step often lose several marks.
4. Answering the Wrong Question
Some questions ask:
volume remaining
Others ask:
volume of product formed
Others ask:
excess reactant remaining
Read the final sentence carefully.
OCR Exam Strategy
Whenever you see a gaseous volume question:
Balance the equation.
Check whether all gases are measured at the same temperature and pressure.
Compare volumes directly.
Identify the limiting reagent.
Calculate the excess.
Check exactly what the question is asking.
This routine works for the vast majority of OCR gaseous volume questions.
Practice Question
Try this one before looking at the answer.
Hydrogen reacts with chlorine.
H₂ + Cl₂ → 2HCl
40 cm³ hydrogen reacts with 30 cm³ chlorine.
Questions
Which reactant is limiting?
How much hydrogen remains?
What volume of hydrogen chloride forms?
Answer
Balanced equation:
1 : 1 : 2
30 cm³ chlorine reacts with : 30 cm³ hydrogen
Hydrogen remaining: 10 cm³
Hydrogen chloride formed: 60 cm³
If you reached those answers without converting into moles, you've understood the method correctly.
Frequently Asked Questions
Do I always compare gas volumes directly?
Only when the gases are measured at the same temperature and pressure. If conditions differ, you'll need to use the appropriate gas calculations.
Why don't I convert cm³ into moles?
Because equal gas volumes contain equal numbers of particles under the same conditions. The balanced equation already provides the correct ratio.
Can OCR ask limiting reagent questions using gas volumes?
Yes. In fact, this is one of the most common ways gaseous volume calculations are assessed.
Key Takeaways
Before moving on, remember these five points:
Balance the equation first.
Gas volumes follow the balanced equation when measured under the same conditions.
Identify the limiting reagent before calculating anything else.
Only calculate the excess reactant after finding the limiting reagent.
Read the question carefully to see whether it asks for the remaining gas, the product formed or the total gas volume.
Mastering this method will help you answer gaseous volume questions more quickly and with greater confidence.
Free OCR A-Level Chemistry Guides
If you're studying OCR A-Level Chemistry and want to avoid the most common mistakes that hold students back, download my free guides:
Year 12 Students:
4 Mistakes That Cause Strong GCSE Students To Struggle In Year 12 Chemistry
Year 13 Students:
4 Mistakes Keeping Capable OCR Chemistry Students Stuck At Grade B Or Below
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