Molar Mass of Gas Calculator

Created by Łucja Zaborowska, MD, PhD candidate
Reviewed by Bogna Szyk and Jack Bowater
Last updated: Jan 18, 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.

Łucja Zaborowska, MD, PhD candidate
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