Reacting-mass and gas-volume questions follow one route: equation, moles of the given substance, ratio, then the quantity asked. The steps never change, only the conversions at each end.
This lesson is part of moles, formulas and equations. It assumes you can already use converting between mass, moles and particles.
What is the four-step route?
Follow the same steps each time.
- Write the balanced equation.
- Convert the given quantity to moles.
- Use the mole ratio to find moles of the required substance.
- Convert those moles to mass or gas volume.
Worked example: mass to volume
13.0 g of zinc reacts with excess dilute hydrochloric acid: Zn + 2HCl → ZnCl₂ + H₂. Find the volume of hydrogen at room conditions. (Zn = 65; 1 mol of gas = 24 dm³.)
Moles of Zn = 13.0 ÷ 65 = 0.200 mol. The ratio Zn : H₂ is 1 : 1, so moles of H₂ = 0.200 mol.
Volume = 0.200 × 24 = 4.80 dm³. Had the question said STP, the answer would be 0.200 × 22.4 = 4.48 dm³.
Worked example: volume to mass
What mass of calcium carbonate gives 1.2 dm³ of carbon dioxide at room conditions with excess acid? CaCO₃ + 2HCl → CaCl₂ + H₂O + CO₂. (Ca = 40, C = 12, O = 16.)
Moles of CO₂ = 1.2 ÷ 24 = 0.050 mol. The ratio CaCO₃ : CO₂ is 1 : 1, so moles of CaCO₃ = 0.050 mol.
Mass = 0.050 × 100 = 5.0 g.
The mistake that costs marks
The common slip is using the wrong molar volume, or forgetting to change cm³ to dm³. Both give an answer that looks reasonable on the calculator.
| Condition in the question | Molar volume to use | What goes wrong |
|---|---|---|
| Room conditions | 24 dm³ mol⁻¹ | Using 22.4 gives 4.48 instead of 4.80 |
| STP | 22.4 dm³ mol⁻¹ | Using 24 gives 4.80 instead of 4.48 |
| Volume given in cm³ | Divide by 1000 first | Answer wrong by a factor of 1000 |
Underline the condition in the question before you start. The mole and stoichiometry steps tool gives a sequence you can follow for your own values.
Check yourself
12.25 g of potassium chlorate is heated: 2KClO₃ → 2KCl + 3O₂. Find the volume of oxygen at room conditions. (K = 39, Cl = 35.5, O = 16.)
Answer
Mr of KClO₃ = 39 + 35.5 + 48 = 122.5, so moles = 12.25 ÷ 122.5 = 0.100 mol.
The ratio KClO₃ : O₂ is 2 : 3, so moles of O₂ = 0.100 × 3 ÷ 2 = 0.150 mol.
Volume = 0.150 × 24 = 3.6 dm³.
What to study next
Concentration questions use the same ratio step with solutions, so continue with calculating concentration from supplied data. Then try the moles practice set.
To have a teacher check your units and ratio on new questions, see online one-to-one Chemistry tuition.