Gas Stoichiometry Calculator
Both volumes shown, because the room is not at STP.
Work out Gas Stoichiometry. Both volumes shown, because the room is not at STP. Every constant is an input, not an assertion.
Calcium carbonate is 100.09
Gas produced at 25 °C and 1 atm
2.444 L
2.239 L at 0 °C and 1 atm
The two volumes differ because gas volume is proportional to absolute temperature: one mole occupies 22.414 L at 0 °C but 24.47 L at 25 °C. Textbook answers are usually quoted at STP while a real experiment happens in the room it is run in, so both are shown here.
How the Gas Stoichiometry Calculator works
The volume of gas a reaction produces, from the reactant mass through the mole ratio and then the ideal gas law. Reported at the conditions you enter and at 0 °C and 1 atm, because textbook answers are quoted at STP while real experiments happen at room temperature.
Also known as: volume of gas produced by a reaction · grams to litres of gas · mass to gas volume stoichiometry · how much co2 does this reaction make
Where the gas law joins the stoichiometry
The mole ratio does the chemistry and the ideal gas law does the physics, and they meet at moles of gas. Convert the reactant mass to moles, apply the coefficient ratio, then substitute into V = nRT/P. Nothing about the gas law knows what reaction produced the gas.
Calcium carbonate decomposing is the standard example: CaCO₃ → CaO + CO₂, a 1:1 ratio. Ten grams at 100.09 g/mol is 0.0999 mol, so 0.0999 mol of CO₂ forms, which at 25 °C and 1 atm occupies 2.44 L. At 0 °C and 1 atm the same amount of gas occupies 2.24 L.
That difference is not an error, and it accounts for a great many hunted-for arithmetic mistakes. Volume scales with absolute temperature, so 298 K against 273 K is a 9.2% difference in volume for exactly the same number of molecules.
Choosing R, and keeping temperature absolute
R takes a different numerical value for every set of units. 0.08206 L·atm/(mol·K) suits pressure in atmospheres and volume in litres. 8.314 J/(mol·K) is the SI value, needing pascals and cubic metres. 62.36 L·mmHg/(mol·K) suits pressure in mmHg. Using one with the units of another is the most common failure on this calculation.
The check is dimensional. Write the units into the expression and cancel them: L·atm/(mol·K) × mol × K ÷ atm leaves litres. If the units do not cancel to a volume, the wrong R was chosen, and the error is caught before it reaches an answer.
Temperature must be in kelvin, without exception. The gas law is built on absolute temperature, and Celsius has an arbitrary zero. Using 25 instead of 298 understates the volume by a factor of twelve, and using a temperature below 0 °C in Celsius returns a negative volume — which at least fails loudly.
Collecting gas over water
A gas collected by displacement of water is saturated with water vapour, so the pressure inside the collecting vessel is the sum of the gas's partial pressure and the vapour pressure of water at that temperature. Dalton's law of partial pressures is what licenses the subtraction.
The correction is not negligible. Water's vapour pressure at 25 °C is 23.8 mmHg, about 3.1% of atmospheric. At 30 °C it is 31.8 mmHg, and at 40 °C, 55.3. Ignoring it overstates the amount of gas actually produced by that percentage, which is larger than most other errors in the experiment.
The other correction people skip is levelling. The water level inside the collecting tube should be brought equal to the level outside before reading the volume, so the gas is at atmospheric pressure. A difference in levels means a difference in pressure, and it has to be converted from millimetres of water to millimetres of mercury — a factor of 13.6 — before it can be applied.
Where to go next
The Gas Stoichiometry question rarely arrives on its own. These are the ones that usually come with it:
- Ideal Gas Law Calculator — PV = nRT, solving for whichever term is missing.
- Stoichiometry Calculator — Grams to moles to moles to grams, through the mole ratio.
- Molar Volume Calculator — STP is ambiguous — 22.414 and 22.711 are both correct.
- Speed Distance Time Calculator — Any one of the three from the other two, in four units.
Frequently asked questions
How do I find the volume of gas a reaction produces?
Convert the reactant mass to moles, apply the mole ratio from the balanced equation, then use PV = nRT to turn moles of gas into a volume at your temperature and pressure.
Why does the answer differ from the textbook?
Usually because the textbook quotes STP and the experiment ran at room temperature. A mole occupies 22.414 L at 0 °C but 24.47 L at 25 °C — a 9% difference that has nothing to do with an arithmetic error.
What value of R should I use?
0.0821 L·atm/(mol·K) when pressure is in atmospheres and volume in litres, or 8.314 J/(mol·K) in SI units. Mixing units with the wrong R is the most common source of wrong answers here.
Does the gas identity change the volume?
No. Avogadro's law says equal numbers of moles occupy equal volumes at the same temperature and pressure, whatever the gas — the mass differs, the volume does not.
Must temperature be in kelvin?
Always. The ideal gas law is built on absolute temperature, and using Celsius gives an answer that is wrong by a factor of hundreds — or a negative volume below 0 °C.
What if the gas is collected over water?
Subtract the vapour pressure of water at that temperature from the total pressure. The collected gas is saturated with water vapour, and ignoring that overstates the partial pressure of the gas you actually made.
How do I convert the answer to a different pressure unit?
1 atm = 760 mmHg = 101.325 kPa = 1.01325 bar = 14.696 psi. Choose R to match the units you are working in rather than converting the answer afterwards — it removes one step where an error can enter.
What is Dalton's law of partial pressures?
The total pressure of a gas mixture is the sum of the pressures each component would exert alone. It is what licenses subtracting water's vapour pressure from a gas collected over water, and what makes mole fraction and pressure fraction the same thing for ideal gases.
What is Gay-Lussac's law of combining volumes?
Gases react in whole-number volume ratios at constant temperature and pressure. Two volumes of hydrogen and one of oxygen give two of water vapour — the same 2:1:2 as the coefficients, because volume is proportional to moles.
Do I need the ideal gas law if both sides are gases?
Not for the ratio. Volume is proportional to moles at fixed temperature and pressure, so the coefficients give the volume ratio directly. The gas law is only needed to turn moles into an absolute volume.
How much error does the ideal assumption introduce?
Under 1% for most gases at ordinary temperature and pressure. It grows near the boiling point and at high pressure — carbon dioxide at 10 atm deviates by several percent, and a van der Waals treatment becomes worthwhile.
What if the reaction produces more than one gas?
Calculate each separately from its own coefficient, then add the volumes if they share a container. Their partial pressures add to the total by Dalton's law, and the total volume is the sum of what each would occupy alone.
Put this calculator on your own site
Free to use, on any site, commercial or not. Paste this where you want it to appear. It is a plain iframe, so it works in WordPress, Squarespace, Wix, Webflow, Ghost and anything else that accepts HTML.
<iframe src="https://www.thecalclibrary.com/embed/gas-stoichiometry-calculator" width="100%" height="640" style="border:1px solid #e2e8f0;border-radius:12px" loading="lazy" title="Gas Stoichiometry Calculator"></iframe>
<p style="font:13px/1.5 system-ui,sans-serif;margin:6px 0 0;color:#64748b">Powered by <a href="https://www.thecalclibrary.com/gas-stoichiometry-calculator" style="color:#64748b">Gas Stoichiometry Calculator</a> from The Calc Library</p>The only condition is that the credit line below the frame stays in place. That one line is what pays for the tool being free — it is how anyone else finds it.
Related calculators
Ideal Gas Law Calculator
PV = nRT, solving for whichever term is missing.
OpenStoichiometry Calculator
Grams to moles to moles to grams, through the mole ratio.
OpenMolar Volume Calculator
STP is ambiguous — 22.414 and 22.711 are both correct.
OpenSpeed Distance Time Calculator
Any one of the three from the other two, in four units.
Open