Hydrogenation Volume Calculator
When hydrogenating an organic compound, you need to know how much hydrogen gas (H2) will be consumed. Each mole of unsaturation (double bond or ring that is reduced) consumes exactly one mole of H2. At STP (0 degrees C, 100 kPa), one mole of H2 occupies 22.414 L. This calculator converts moles of substrate and equivalents of H2 consumed per mole of substrate into the total volume of H2 at STP. This is useful for sizing gas reservoirs, monitoring Parr apparatus pressure drop, or planning large-scale hydrogenations.
Hydrogenation volume formula
Moles H2 = moles substrate x equivalents
Volume (L) = moles H2 x 22.414 L/mol
The molar volume of an ideal gas at STP (0 degrees C, 100 kPa) is 22.414 L/mol by the IUPAC definition. Multiply moles of hydrogen consumed by this value to get the volume at STP. To correct for a different temperature and pressure, apply the ideal gas law: V = nRT/P.
Common hydrogenation scenarios
- Alkene (1 double bond): 1 equivalent H2 consumed per mole; product is the saturated alkane.
- Alkyne to alkene (Lindlar catalyst or partial hydrogenation): 1 equivalent H2, gives cis-alkene.
- Alkyne to alkane: 2 equivalents H2 consumed per mole of alkyne.
- Aromatic ring (Birch conditions): 3 equivalents H2 to fully reduce benzene to cyclohexane.
- Nitrile to amine: 2 equivalents H2 consumed per mole under high-pressure conditions.
Hydrogenation volume: frequently asked questions
What is the molar volume of an ideal gas at STP?
At IUPAC standard temperature and pressure (0 degrees C and 100 kPa), one mole of an ideal gas occupies 22.414 L. This is the molar volume used to convert between moles of gas and volume in hydrogenation calculations.
How many equivalents of H2 does a double bond consume?
One mole of H2 is consumed per mole of double bond (alkene or carbonyl with catalytic hydrogenation). An alkyne consumes 2 moles of H2 if fully reduced to an alkane, or 1 mole if reduced to an alkene using a Lindlar catalyst.
Why does this matter in the laboratory?
In a Parr hydrogenation apparatus or balloon setup, you need to know how much H2 to supply. Calculating the expected uptake volume helps confirm reaction completion and avoid excess pressure.
What is STP exactly?
STP as defined by IUPAC since 1982 is 0 degrees C (273.15 K) and 100 kPa (approximately 0.987 atm). The older NIST definition used 1 atm (101.325 kPa). For practical gas volume calculations the difference is less than 1.4%, so 22.4 L/mol is used as the standard molar volume.
How do I calculate moles of substrate?
Moles = mass (g) / molar mass (g/mol). For example, 1.00 g of cyclohexene (MW 82.14 g/mol) gives 0.01217 mol, which consumes 0.01217 mol H2, or 0.01217 x 22.414 = 0.273 L at STP.
Official sources
- NIST: Molar volume of ideal gas at STP.
- IUPAC: Quantities, Units and Symbols in Physical Chemistry (Green Book).
Reviewed by the CalculatorHub team, edited by James Graham, 15 June 2026. See our methodology.