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Molar Mass of Gas Calculator

Chemistry

Calculate molar mass of a gas from its density using M = dRT/P, or from PVT measurements using PV=nRT. Enter gas density at known temperature and pressure.

1.429
0
1

Molar Mass (g/mol)

32.03
Density at STP (g/L)
1.429
Molar Volume at STP (L/mol)
22.414

This calculator computes your Molar Mass (g/mol), Density at STP (g/L), Molar Volume at STP (L/mol) from the values you enter.

Inputs
Gas Density (g/L)Temperature (°C)Pressure (atm)
Outputs
Molar Mass (g/mol)Density at STP (g/L)Molar Volume at STP (L/mol)

What is a Gas Molar Mass?

The Molar Mass of Gas Calculator determines the molar mass of a gas or vapour from its measured density at known temperature and pressure, using M = dRT/P derived from the ideal gas law. Enter gas density (g/L), temperature (°C), and pressure (atm) to get the molar mass (g/mol), density at STP, and molar volume.

The relationship M = dRT/P is the most direct experimental method for determining molar mass of gases and volatile liquids. By measuring the mass of a known volume of gas at known temperature and pressure, the molar mass follows immediately. This connects to the broader toolset of gas law calculations: the STP Calculator converts between volumes and moles at STP; the Rate of Effusion Calculator provides Graham's law approach to molar mass from effusion rates; and the Molar Mass Calculator computes molar mass from atomic composition for known compounds.

The calculator also outputs density at STP (0°C, 1 atm), allowing comparison of all gases on a common basis. Gases lighter than air (density at STP < 1.293 g/L, molar mass < 28.97 g/mol) are listed in the worked example — a safety-relevant distinction for industrial gas handling.

How to use this Gas Molar Mass calculator

  1. Measure the gas density by weighing a known volume of gas at known temperature and pressure, or look up the density from a databook. Enter in Gas Density (g/L).
  2. Enter the Temperature in °C at which the density measurement was made.
  3. Enter the Pressure in atm at which the density was measured. Standard atmospheric pressure = 1 atm.
  4. Read Molar Mass (g/mol) — identify the gas by comparing to known molar masses.
  5. Check Density at STP for comparison to tabulated standard gas densities (O₂ at STP = 1.429 g/L, N₂ = 1.250 g/L, etc.).

Formula & Methodology

Molar mass from ideal gas density:

M = dRT/P R = 0.082057 L·atm/(mol·K) T = temperature in Kelvin = T(°C) + 273.15

STP density:

d_STP = M / 22.414    (in g/L)

Worked example — identifying an unknown gas:

An unknown gas has density 1.965 g/L at 0°C and 1 atm (i.e., at STP). Identify the gas.

M = d × RT/P = 1.965 × 0.082057 × 273.15 / 1   = 1.965 × 22.414   = 44.04 g/mol

Molar mass = 44 g/mol → likely CO₂ (44.01 g/mol) or N₂O (44.01 g/mol). In practice, additional properties (IR spectrum, chemical reactivity, odour) distinguish between isobaric gases. At STP, CO₂ has density 1.965 g/L — matching the default input in this calculator.

Frequently Asked Questions

The molar mass of a gas (M) can be calculated from its density (d), temperature (T), and pressure (P) using the ideal gas law rearranged as M = dRT/P, where d is in g/L, R = 0.082057 L·atm/(mol·K), T is in Kelvin, and P is in atm. This follows from PV = nRT rearranged as PM/d = RT: since d = m/V and M = m/n, we get PV = (m/M)RT → P(M/d) = RT → M = dRT/P.
At STP (0°C, 1 atm), the density of any ideal gas is d_STP = M/22.414 g/L, where M is the molar mass in g/mol. Common gas densities at STP: H₂ = 0.0899 g/L, He = 0.178 g/L, N₂ = 1.250 g/L, O₂ = 1.429 g/L (matching the default in this calculator), CO₂ = 1.965 g/L, Cl₂ = 3.170 g/L. Gases denser than air (1.29 g/L at STP) accumulate near the ground; lighter gases rise.
From PV = nRT: n = PV/RT. Molar mass M = mass/moles = mass × RT/(PV) = (mass/V) × RT/P = d × RT/P. This rearrangement M = dRT/P is the key equation. Alternatively, at STP: M = d_STP × 22.414 (since one mole occupies 22.414 L at STP). Both approaches are equivalent; the M = dRT/P form is more general and applies at any temperature and pressure.
Enter the gas density in g/L, the temperature in °C (at which the density was measured), and the pressure in atm. The calculator applies M = dRT/P and returns the molar mass in g/mol, the density at STP (0°C, 1 atm), and the molar volume at STP (22.414 L/mol for ideal gas).
Dry air is a mixture: 78.09% N₂ (M=28), 20.95% O₂ (M=32), 0.93% Ar (M=40), 0.04% CO₂ (M=44). Average molar mass = 0.7809×28 + 0.2095×32 + 0.0093×40 + 0.0004×44 = 21.87 + 6.70 + 0.37 + 0.02 = 28.97 g/mol. At STP, air density = 28.97/22.414 = 1.293 g/L. Gases with M < 28.97 g/mol are lighter than air (H₂, He, CH₄, NH₃, N₂, CO); gases with M > 28.97 are heavier (O₂, Ar, CO₂, Cl₂).
The ideal gas law (and M = dRT/P) is most accurate at low pressure and high temperature. Deviations increase at high pressure and low temperature, near the critical point, and for gases with strong intermolecular forces. CO₂ deviates noticeably above 50 atm; NH₃ deviates at moderate pressures due to hydrogen bonding; noble gases are closest to ideal. For practical molar mass determination at atmospheric pressure and room temperature, ideal gas law errors are less than 1% for most gases.
Yes — the method applies to any gaseous substance, including vapours of volatile liquids. This is the Victor Meyer method: a known mass of the liquid is vaporised in a known volume at elevated temperature and the gas density (or pressure) is measured. The molar mass of volatile organic compounds (ethanol, acetone, benzene) can be determined this way. At 100°C and 1 atm, ethanol vapour has d ≈ 1.01 g/L; M = dRT/P = 1.01 × 0.082057 × 373.15 / 1 = 30.9 g/L — close to the true 46.07 g/mol, suggesting incomplete vaporisation or association.
Graham's law of effusion states that effusion rate ∝ 1/√M. By measuring the effusion rate of an unknown gas relative to a reference gas, the unknown molar mass can be calculated: M_unknown = M_ref × (r_ref/r_unknown)². This is an alternative to density measurement for molar mass determination, especially for trace gases or gases at non-atmospheric pressure. Use the [Rate of Effusion Calculator](/rate-of-effusion-calculator/) for Graham's law calculations.
Vapour density (D_v) is the density of a gas or vapour relative to the density of hydrogen gas at the same conditions: D_v = d_gas / d_H₂. Since d_H₂ at STP = 0.0899 g/L and the ideal gas density is proportional to molar mass: D_v = M_gas / M_H₂ = M_gas / 2. Therefore M_gas = 2 × D_v. This relation was historically important before precise atomic masses were available. Vapour density is no longer commonly used but appears in older textbooks and some Indian chemistry curricula.
Yes — molar mass determines gas density and therefore buoyancy, storage requirements, and safety considerations. LPG (primarily propane C₃H₈, M=44, and butane C₄H₁₀, M=58) is denser than air (M=29) — leaking LPG sinks to the floor and accumulates, creating explosion risk. Methane (CNG, M=16) is lighter than air and dissipates upward, making CNG leaks less dangerous in well-ventilated spaces. Indian gas safety standards (BIS, PESO) account for these molar mass-based density differences in storage and facility design.