Gaseous Volume Exam Question (MCQ)
- Sep 17, 2019
- 2 min read
Updated: 6 days ago
Prefer to watch rather than read?
This video works through a multiple-choice gaseous volume question using simple mole ratios and Avogadro's law.
Many A-Level Chemistry students understand balancing equations but become confused when gases are measured in cm³ rather than moles.
Fortunately, at the same temperature and pressure, gas volumes react in exactly the same ratio as the balanced equation.
This makes many gaseous volume questions much quicker than students expect.
Exam 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
This tells us the reacting gas volumes are in the ratio:
1 : 1
Step 2 – Identify the limiting reagent
30 cm³ Xe would require: 30 cm³ F₂
Only 20 cm³ F₂ is available.
Fluorine is the limiting reagent and therefore no Fluorine remains.
Step 3 – Calculate how much xenon reacts
20 cm³ F₂ reacts with:20 cm³ Xe
Step 4 – Calculate the amount of Xe gas remaining
Started with: 30 cm³ Xe
Reacted: 20 cm³ Xe
Remaining: 10 cm³ Xe
Answer
10 cm³ of xenon remains.
Step 4 – Calculate the amount of XeF₂ formed
1:1 ratio with Fluorine
Therefore 20 cm³ XeF₂ formed
Step 5 – Calculate the total mount of gas
10 cm³ of xenon + 20 cm³ XeF₂ = 30 cm³
Exam Tip
Whenever a gaseous volume question states that gases are measured at the same temperature and pressure, you can compare gas volumes directly using the balanced equation.
There is no need to convert cm³ into moles first.
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