1.7.1Physical Chemistry
Reacting Volume Calculations — Edexcel AS Chemistry Revision Notes & Topical Questions
5 min read 6 parts Lesson 30 of 42
Part 1 of 6Gas Volumes and Moles
Equal volumes
- Equal volumes of different gases at the same temperature and pressure contain the same number of molecules.
Molar gas volume
- At room temperature and pressure one mole of any gas occupies about 24.0 dm³.
Volume ratios
- For gases measured under the same conditions, the ratio of volumes equals the ratio of coefficients in the equation.
Limits
- Volume ratios only apply to gases, not to liquids or solids formed in the reaction.
Part 2 of 6The Ideal Gas Equation
The Ideal Gas Equation
Equation
- pV = nRT links pressure, volume, amount and temperature of a gas.
Units
- Use p in pascals, V in m³, T in kelvin and R = 8.31 J mol⁻¹ K⁻¹.
Converting
- To change °C to K add 273, and to change dm³ to m³ divide by 1000.
Use
- Rearrange to find any one quantity when the other three are known.
| Quantity | Symbol | SI unit |
|---|---|---|
| Pressure | p | Pa |
| Volume | V | m³ |
| Amount | n | mol |
| Temperature | T | K |
Part 3 of 6Working Through a Gas Volume Question
1
Write the balanced equation and note which species are gases at the conditions given.
2
Compare the volumes of gases using the coefficients when conditions are the same.
3
If a mass is given, convert it to moles first and then use molar gas volume or pV = nRT.
4
Account for gases that remain at the end, such as unused oxygen.
5
Use the information that cooling removes steam and potassium hydroxide removes CO₂ to interpret contractions.
Part 4 of 6Typical Contraction Questions
Initial volume
- Record the volume of hydrocarbon and oxygen before they react.
After cooling
- Water condenses to a liquid, so it is not counted as a gas.
After alkali
- Carbon dioxide is absorbed by aqueous sodium hydroxide or potassium hydroxide.
Remaining gas
- The residue is excess oxygen, which can be used to work out how much was consumed.
Part 5 of 6Worked Example
Question
- 10 cm³ of methane is burned in 50 cm³ of oxygen, all volumes measured at the same conditions.
- CH₄(g) + 2O₂(g) → CO₂(g) + 2H₂O(l).
- Find the total gas volume after cooling and after shaking with aqueous KOH.
- Methane reacts with twice its volume of oxygen: 10 × 2 = 20 cm³ of O₂ used.
- Oxygen left over = 50 − 20 = 30 cm³.
- Carbon dioxide formed equals the volume of methane, 10 cm³; water is a liquid.
- After cooling the gas volume = 30 + 10 = 40 cm³.
- KOH absorbs CO₂, so the final volume = 30 cm³.
Answer
- 40 cm³ after cooling;
- 30 cm³ after shaking with KOH.
Part 6 of 6Exam Checklist & Tips
1.7.1 – Key Points for the Exam
- Molar gas volume Volume occupied by one mole of gas, 24.0 dm³ at RTP.
- Ideal gas equation pV = nRT.What the symbols mean: p = pressure (Pa) · V = volume (m3) · n = amount of gas (mol) · R = 8.31 J mol−1 K−1 · T = temperature (K = °C + 273)
- RTP Room temperature and pressure, about 298 K and 100 kPa.
- Contraction Decrease in total gas volume after a reaction.
Common Mistakes
- Using volume ratios when gases are measured at different temperatures or pressures.
- Counting liquid water as a gas.
- Forgetting to convert temperature to kelvin.
- Using cm³ in pV = nRT instead of m³.
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