Molar Mass of Gas Calculator

M = mRT / (PV), a rearrangement of the ideal gas law PV = nRT using n = m/M (moles equals mass divided by molar mass). Here m is the measured mass of gas in grams, R is the gas constant, T is absolute temperature in kelvin, P is pressure in atm, and V is volume in litres.

This calculator determines molar mass experimentally, from physically measured pressure, volume, temperature, and mass of an unknown gas sample, without needing to know its chemical formula in advance. Calculating molecular weight by summing atomic masses (as the site's Molecular Weight Calculator does) requires already knowing the formula, which is the opposite starting point.

R = 0.082057 L·atm/(mol·K), the standard gas constant used when pressure is in atmospheres and volume is in litres. This matches the common convention in general chemistry courses and lab manuals for experimental molar mass determination.

The Dumas bulb method is a classic laboratory technique for finding the molar mass of a volatile liquid. A small amount of liquid is placed in a flask of known volume, heated until it fully vaporises and drives out the air, then sealed and cooled. The mass of the condensed vapour, along with the flask's volume, the vaporisation temperature, and atmospheric pressure, are plugged into M = mRT/(PV) to find the molar mass.

The ideal gas law, and this rearranged form, is derived from the kinetic theory of gases, where temperature must be measured on an absolute scale starting at absolute zero. Using Celsius or Fahrenheit directly would give a wrong (and sometimes negative or infinite) result, since those scales do not start at true zero energy.

It is generally accurate to within a few percent for gases at moderate temperature and pressure, since it relies on the ideal gas approximation. Accuracy degrades at high pressure or low temperature, where real gas behaviour (intermolecular forces, finite molecular volume) deviates more from the ideal gas law's assumptions.

Yes, approximately. Once you calculate the molar mass, compare it to known molar masses of common gases or compounds: for example, a result near 44 g/mol suggests carbon dioxide (44.01 g/mol) or propane (44.10 g/mol), while a result near 64 g/mol suggests sulfur dioxide (64.07 g/mol). Molar mass alone cannot distinguish between compounds with very similar values, so it is usually combined with other tests.

Convert first: divide millilitres by 1000 to get litres, and divide mmHg by 760 to get atm (since 760 mmHg = 1 atm exactly). Entering unconverted values will give an incorrect molar mass, since R = 0.082057 only pairs correctly with atm and litres.

Yes. The formula M = mRT/(PV) is derived directly from the ideal gas law, which assumes negligible intermolecular forces and negligible molecular volume. This is a good approximation for most gases at room temperature and moderate pressure, but becomes less accurate for gases close to condensation or at very high pressure.

The Ideal Gas Law Calculator solves the general equation PV = nRT for any one of pressure, volume, moles, or temperature given the other three. This calculator is a dedicated, single-purpose tool for the specific experimental task of finding molar mass from measured mass, pressure, volume, and temperature, which requires the m/M substitution for moles.

For meaningful accuracy, mass should be measured to at least three significant figures (a good laboratory balance reads to 0.001 g or better), and volume, temperature, and pressure should each be recorded as precisely as your equipment allows, since small measurement errors in any one variable propagate directly into the calculated molar mass.