Global warming potential
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The Global warming potential numbers are expressed by mass: The GWP of CH4 is 84, meaning that 1kg of CH4 has the same global warming potential as 84 kg CO2.
But the concentrations you'll see of CO2, CH4, etc are usually expressed in parts per million (PPM) or parts per billion (PPB), ie molar fractions.
Here I've calculated the Global Warming Potential numbers for gasses of the same molar fraction (PPM or PPB).
To start, let's calculate the GWP by molar fraction of CH4 the hard way, by calculating the mass of 1 PPM CH4 in 1 m3 of gas, multiplying that by 84 to calculate the mass of the equivalent CO2, converting that back to a m3 of gas at 25 °C contains 1/0.024789598=40.34 mol gas.
