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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.

Reviewed by the thecalcu.com team · Last updated January 10, 2026

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.

Why Use a Molar Mass of Gas Calculator?

The rearrangement from PV = nRT to M = dRT/P requires two algebraic steps and careful handling of units (d in g/L, R in L·atm/(mol·K), T in K, P in atm). Using the wrong R (8.314 J/(mol·K) instead of 0.082057 L·atm/(mol·K)) gives results 101.3 times larger. This calculator uses the correct R for the L·atm system internally.

For NCERT Class 11 Chapter 5 exam problems, calculating molar mass from gas density at specified conditions is a standard problem type. The formula M = dRT/P and its derivation from PV = nRT are explicitly required knowledge.

Who Should Use This Calculator?

Class 11 and 12 chemistry students studying the States of Matter chapter, where molar mass determination from gas density is a standard problem type for CBSE boards and JEE preparation.

Analytical chemists determining molar masses of unknown gases or volatile compounds via gas density measurement (Victor Meyer method, Dumas method).

Industrial gas engineers computing expected gas densities from molar masses for dispersion modelling, safety classification (lighter or heavier than air), and storage system design.

Environmental scientists computing molar masses of atmospheric pollutants and greenhouse gases for emissions inventory and dispersion calculations.

Chemical engineering students studying thermodynamics and transport properties where gas density and molar mass are fundamental inputs.

What Insights Does the Molar Mass of Gas Calculator Give You?

Molar Mass (g/mol) is the primary output, the molecular weight of the gas or vapour. Compare to known molar masses to identify the gas or verify measurement: O₂ = 32, N₂ = 28, CO₂ = 44, CH₄ = 16, SO₂ = 64, etc.

Density at STP (g/L) converts the measured density to the standard reference conditions (0°C, 1 atm), allowing comparison with tabulated gas densities. Equal to M/22.414.

Molar Volume at STP (L/mol) is always 22.414 L/mol for an ideal gas, shown here as a confirmation of the ideal gas reference value used in the calculation.

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.).
Show formula & methodology ↓Show less ↑

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

What is the molar mass of a gas and how is it calculated from density?
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.
What is the density of a gas at STP?
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.
How does the ideal gas law give molar mass from density?
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.
How do I use the Molar Mass of Gas Calculator?
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).
What is the molar mass of air and why is it relevant?
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).
What real gases deviate significantly from ideal behaviour?
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.
Can this calculator determine the molar mass of a vapour?
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.
How is Graham's law related to molar mass of gas?
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.
What is vapour density and how does it relate to molar mass?
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.
Is molar mass of gas relevant to industrial process design in India?
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.