Gas Stoichiometry
Gas stoichiometry is not a new method, it is the 4-step method with one extra conversion step added. When a gas is given or asked for, you convert between volume and moles using molar volume, then proceed as normal. The only trap is choosing the right molar volume for the stated conditions.
Practise this lesson
Four printable worksheets that build from the foundations up to exam-style questions, start at whatever level suits you.
Imagine you burn a piece of magnesium ribbon in oxygen: 2Mg + O₂ → 2MgO. If you know the mass of magnesium used, what extra step would you need to find the volume of oxygen gas consumed, and why can't you just use the same 4-step method you already know?
Gas Stoichiometry Formulas
V = n × molar volume (moles → gas volume)
STP (0°C, 100 kPa): molar volume = 22.71 L/mol
RTP (25°C, 100 kPa): molar volume = 24.8 L/mol
Gas laws (T in kelvin, T(K) = T(°C) + 273.15):
Boyle: P₁V₁ = P₂V₂ | Charles: V₁/T₁ = V₂/T₂ | Gay-Lussac: P₁/T₁ = P₂/T₂
Combined: P₁V₁/T₁ = P₂V₂/T₂
Ideal gas law: PV = nRT, R = 8.314 (kPa·L·mol⁻¹·K⁻¹)
Key facts
- STP = 0°C, 100 kPa → 22.71 L/mol (NESA standard)
- RTP = 25°C, 100 kPa → 24.8 L/mol
- Temperature in gas laws must be in kelvin: T(K) = T(°C) + 273.15
- Ideal gas law: PV = nRT, R = 8.314 (kPa·L·mol⁻¹·K⁻¹)
Concepts
- Boyle, Charles, Gay-Lussac and Avogadro each hold two variables constant
- The combined and ideal gas laws unify them into one relationship
- Molar volume only applies at one fixed set of conditions
- Gas stoichiometry links gas volumes to moles via the gas laws
Skills
- Convert between °C and K and apply P₁V₁/T₁ = P₂V₂/T₂
- Solve for any one of P, V, n or T using PV = nRT
- Find a gas volume produced or consumed in a reaction at any conditions
- Read and interpret P–V, V–T and P–T graphs
Avogadro's law states that equal volumes of all gases at the same temperature and pressure contain equal numbers of molecules. This means 1 mole of any gas, regardless of what it is, occupies the same volume under the same conditions.
STP, Standard Temperature and Pressure
- Temperature: 0°C (273 K)
- Pressure: 100 kPa
- Used when question says "STP" or "0°C, 100 kPa"
- Current NESA standard. Note: 22.4 L/mol is the older value at 0°C and 1 atm (101.325 kPa), you may see it in older resources. NESA uses 22.71 L/mol at 0°C and 100 kPa.
RTP, Room Temperature and Pressure
- Temperature: 25°C (298 K)
- Pressure: 100 kPa
- Used when question says "RTP", "room conditions", or "25°C"
- More realistic for laboratory experiments
If a gas volume is the input (given) → use Step 0: n = V ÷ MV, then proceed to Step 3 directly.
If a gas volume is the output (asked for) → complete Steps 1–3 normally, then use Step 5: V = n × MV.
If both input and output are gases → use Step 0 AND Step 5.
Avogadro's law: equal volumes of all gases at the same T and P contain equal numbers of molecules. Molar volume: STP (0 °C, 100 kPa) = 22.71 L mol⁻¹; RTP (25 °C, 100 kPa) = 24.8 L mol⁻¹. To find moles from gas volume: n = V ÷ Vm. Always identify conditions before choosing a Vm value.
Pause, copy the highlighted rule and values into your book before moving on.
Odd one out: three of these conditions all use 24.8 L/mol as the molar volume. Which one doesn't belong?
Molar volume only works at one fixed set of conditions. The gas laws let you predict what a gas does when pressure, volume or temperature change. Each simple law holds two of the four quantities (P, V, T, n) constant and links the other two.
- Constant T, n
- P and V inversely proportional
- P₁V₁ = P₂V₂
- Constant P, n
- V directly proportional to T
- V₁/T₁ = V₂/T₂
- Constant V, n
- P directly proportional to T
- P₁/T₁ = P₂/T₂
- Constant P, T
- V directly proportional to n
- V₁/n₁ = V₂/n₂
Combine the three P–V–T laws into one relationship for a fixed amount of gas:
P₁V₁ / T₁ = P₂V₂ / T₂
Bring in the amount of gas and you get the ideal gas law:
P V = n R T
where R = 8.314 J mol⁻¹ K⁻¹. Because 1 J = 1 kPa·L, you can use R = 8.314 with P in kPa, V in L, T in K directly. The molar volume you already use (22.71 L/mol at STP) is just PV = nRT solved at 0 °C and 100 kPa.
Gas laws (T always in kelvin, T(K) = T(°C) + 273.15): Boyle P₁V₁ = P₂V₂ (constant T); Charles V₁/T₁ = V₂/T₂ (constant P); Gay-Lussac P₁/T₁ = P₂/T₂ (constant V); Avogadro V ∝ n. Combined: P₁V₁/T₁ = P₂V₂/T₂. Ideal gas law: PV = nRT, R = 8.314 (kPa·L·mol⁻¹·K⁻¹). Molar volume is PV = nRT evaluated at one fixed set of conditions.
Pause, copy the five gas-law equations into your book before moving on.
Quick check: A sealed, rigid steel cylinder of gas is heated. Which gas law predicts how its pressure changes, and what happens?
A weather balloon holds 15.0 L of helium at ground level (100 kPa, 27 °C). It rises until the pressure is 40.0 kPa and the temperature is −23 °C. Find the new volume.
Calculate the volume occupied by 0.250 mol of oxygen gas at 30 °C and 95.0 kPa.
Worked examples · reveal as you go
What volume of CO₂ is produced at STP when 25.0 g of CaCO₃ decomposes? CaCO₃ → CaO + CO₂. (Ca=40.078, C=12.011, O=15.999)
What volume of O₂ at RTP is required to completely burn 0.500 mol of C₂H₆? 2C₂H₆ + 7O₂ → 4CO₂ + 6H₂O.
What mass of Zn is needed to produce 3.72 L of H₂ at RTP? Zn + 2HCl → ZnCl₂ + H₂. (Zn = 65.38)
In 2H₂ + O₂ → 2H₂O (gas), what volume of H₂O vapour forms from 4.00 L of H₂ at constant temperature and pressure?
Click two steps to swap them. Order the gas-stoichiometry method to solve: what volume of CO₂ is produced at STP when 25.0 g of CaCO₃ decomposes? (CaCO₃ → CaO + CO₂)
- Apply the mole ratio CaCO₃ : CO₂ = 1 : 1, so n(CO₂) = 0.2498 mol.
- Identify the conditions (STP → use 22.71 L/mol) and confirm the equation is balanced.
- Final answer: V(CO₂) = 5.67 L at STP.
- Convert mass to moles: MM(CaCO₃) = 100.09; n(CaCO₃) = 25.0 ÷ 100.09 = 0.2498 mol.
- Convert moles of gas to volume: V = n × Vₘ = 0.2498 × 22.71.
Common errors · the 3 traps that cost marks
Using 22.71 L/mol for RTP conditions (or 24.8 for STP)
This is the single most tested trap in gas stoichiometry. The question will almost always specify conditions, read for "STP", "standard conditions", "0°C" (use 22.71), or "RTP", "room temperature", "25°C" (use 24.8). Using the wrong value gives an answer that is off by a factor of 24.8 ÷ 22.71 = 1.107, a 10.7% error that will cost marks even if all other steps are correct.
✓ Fix: Before any calculation, underline the conditions stated in the question. Write "STP → 22.71" or "RTP → 24.8" at the top of your working before you start.
Forgetting to convert mass to moles before applying the mole ratio
When a solid reactant mass is given and a gas volume is asked for, students sometimes skip Step 2 (n = m ÷ MM) and go straight from mass to volume using the molar volume. This is wrong, molar volume converts moles to litres, not grams to litres. You must convert mass → moles first, then apply the ratio, then convert moles → volume.
✓ Fix: Always go mass → moles → ratio → moles of gas → volume. Never skip the mass-to-moles step, even when the question asks for a gas volume.
Applying the gas volume ratio shortcut when reactants are not all gases
The volume ratio shortcut (volume ratio = coefficient ratio) only applies when ALL species in the comparison are gases at the same temperature and pressure. In CaCO₃ → CaO + CO₂, the CaCO₃ and CaO are solids, only CO₂ is a gas. You cannot say "1 L of CaCO₃ produces 1 L of CO₂", solids don't have volumes in this sense. The shortcut works only for reactions like H₂ + Cl₂ → HCl, where all species are gases.
✓ Fix: Use the volume ratio shortcut only when every reactant and product you're comparing is explicitly a gas in the question. If any solid or liquid is involved, use the full 4-step method.
Quick-fire practice · 5 reps +2 XP per reveal
What volume does 2.0 mol of an ideal gas occupy at 25 °C and 100 kPa? (Vₘ = 24.79 L mol⁻¹)
How many moles of gas are in 12.4 L at 25 °C and 100 kPa? (Vₘ = 24.79 L mol⁻¹)
For N₂ + 3H₂ → 2NH₃, what volume of H₂ reacts with 10 L of N₂ (same temperature and pressure)?
What volume of CO₂ (25 °C, 100 kPa) forms when 0.20 mol of CaCO₃ decomposes? (CaCO₃ → CaO + CO₂)
Find the mass of 6.0 L of O₂ at 25 °C and 100 kPa. (Vₘ = 24.79 L mol⁻¹, M(O₂) = 32.00)
At the start of this lesson, you thought about what extra step is needed to find the volume of gas consumed or produced in a stoichiometry problem.
The answer is: gas stoichiometry is simply the 4-step method with one extra conversion. Before Step 1 (if gas volume is given), use n = V ÷ molar volume to convert to moles. After Step 3 (if gas volume is the answer), use V = n × molar volume. The molar volume is 22.71 L/mol at STP (0°C) or 24.8 L/mol at RTP (25°C), always read the conditions in the question before choosing.
Reflect: how did your initial thinking compare to what you've learned?
Write a reflection in your workbook.