Molar Mass of Gas Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Bogna Szyk and Jack Bowater
Last updated: Jul 17, 2023

This molar mass of gas calculator is a tool that uses the ideal gas law formula to work out an unknown gas' molar mass and the number of moles of it present.

Read on to learn more about the ideal gas law, moles, and examples & tips on how to resolve chemical equations.

This calculator allows you to input the data you have in any order you wish – so don't stress if you know the molar mass but not the pressure! Our extensive range of units will enable you to calculate the desired value without hassle; you won't have to leave this page! It also provides you with an opportunity to change all the values on the go.

You can also use a tool to convert the mass concentration of any solution to its molar concentration.

How to calculate molar mass of a gas?

You need the following data about the gas:

  • Pressure (most commonly used units: atm, kPa);
  • Temperature (°C, °F, K);
  • Volume of the gas (ml, L, dm³, m³); and
  • Mass (not required for number of moles calculations).

Our gas law calculator uses the following equations:

The modified ideal gas law formula:

Moles = (Pressure × Volume) / (0.0821 × Temperature)

If you want to work it out yourself, without the molar mass of gas calculator, be careful with the units! This particular equation uses a constant of 0.0821, which is intended for the following units:

  • Pressure = Atmosphere (atm)

  • Volume = Liters (L)

  • Temperature = Kelvin (K)

The molar mass equation

Molar mass = Mass / Moles

It's as simple as that! Recommended units:

  • Mass = grams (g)

But your mass isn't given in grams? Don't worry; why don't you take some time to discover how to properly convert between different densities and weights?

Your result will show in g/mol.

The calculated value is numerically identical to 1 u (or 1 Da = Dalton, used in biochemistry). A Dalton is a unit of atomic mass equal to the mass of 1/12 of a particle of carbon ¹²C.

Molar mass and moles

A mole is a unit used for measuring matter. One mol contains exactly 6.02214076×10²³ elementary particles (this number is called the [Avogadro's Number]). When we use g/mol, we describe the weight of one mole of a given substance.

Molar mass is often confused with atomic or molecular mass. Although their values are identical, they describe different quantities.

  • Atomic mass informs us about the mass of a unit of a given substance
  • Molecular mass is equal to the mass of one molecule of a substance.

It's good to know your Periodic Table; have you ever tried to calculate an atom's atomic mass?

The use of ideal gas law calculators

The molar mass of gas is not the only thing we can calculate with the ideal gas law!

There are plenty of chemistry-based queries that can be solved by some form of the original ideal gas law. That's why we use the combined gas law calculator (a.k.a. PV-NRT calculators). With just a few transformations, we can use this formula to determine all the properties of a given gas in three types of processes: isobaric, isochoric, and isothermal.

Below you will find all of the most essential, ready-to-go equations used in all those calculations, along with a quick explanation.

Ideal gas law formula:

PV = nRT,


  • P – Pressure;

  • V – Volume;

  • T – Temperature;

  • n – Number of moles of the substance;

  • R – The ideal gas constant = 8.314 J/(mol·K) = 0.082 (L·atm)/(mol·K).

    (R is equal to the Avogadro's constant multiplied by the Boltzmann constant)

Modifications to the ideal gas equation:

Always remember that the nR part of any of these equations is constant – it means it may be crossed out when you transform the formula. Depending on the process, you may also cross out one of the following variables: T, V, P. Try to keep your notes as simple as possible!

  1. Boyle's law – The formula used when dealing with an isothermal process (a process where the temperature does not change):

    • n, R, and P are constant!

    • PV = is constant

    • P₁V₁ = P₂V₂

    More knowledge never hurts anyone. So how about giving the Boyle's law calculator a try?

  2. Charles's law – The formula used when dealing with an isobaric process (a process where the temperature does not change):

    • n, R, and P are constant!

    • T₁/V₁ = T₂/V₂ or

    • V₁T₂ = V₂T₁

    We have a stand-alone Charles' law calculator if you are interested in knowing more.

  3. Gay-Lussac's law – The formula used when dealing with an isochoric process (a process where the temperature does not change):

    • n, R, and V are constant!

    • T₁/P₁ = T₂/P₂ or

    • P₁T₂ = P₂T₁

    You might wanna check out our Gay-Lussac's law calculator.


How do I calculate the molar mass of a gas?

To calculate the molar mass of a gas, follow these steps:

  1. Use the ideal gas law formula to find the number of moles of gas:

    number of moles = PV / RT

    When substituting values, be sure to use consistent units.

  2. Once you have the number of moles, find the molar mass by calculating the ratio between the mass of the gas and the number of moles:

    molar mass = mass / number of moles

    Your result should be in units of mass per mol (g/mol, kg/mol).

What's the molar mass of 3.66 mol of nitrogen gas?

The molar mass of N2 is 28.0134 g/mol. Obtain this by adding the molar masses of the two nitrogen atoms in the molecule (14.0067 g/mol each). Since N2 is a known substance, the number of moles is unnecessary to find its molar mass.

For a mixture of gases, use the ideal gas law formula to find the number of moles and the weighted mass:

molar mass = mass / number of moles

Is the molar mass the same as the molecular weight of a gas?

No, molar mass, and molecular weight are different. Even though they have the same numerical value, molar mass is the mass of one mole of a substance, usually in grams (g/mol). While molecular weight is the mass of one molecule of a substance in atomic mass units (amu).

What's the mass of 0.560 moles of chlorine gas?

The mass of 0.560 moles of chlorine gas Cl2 is 39.71 g. To obtain this value, follow these steps:

  1. Determine the molar mass of the gas. Since chlorine has a molar mass of 35.453 g/mol on the periodic table, the molar mass of the chlorine gas Cl2 is twice this value. This is 70.906 g/mol.

  2. Use the molar mass formula to calculate the mass:

    mass = molar mass × number of moles

  3. Substitute the known values:

    mass = 70.906 g/mol × 0.560 mol = 39.71 g

Łucja Zaborowska, MD, PhD candidate
Molar mass of the gas
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