Write down all of the information that the problem gives you. To calculate fugacity, you will need the temperature of the reaction or system and the concentration of the gas for which you want to calculate fugacity. Please note that you only need the concentration of the gas that you need the fugacity for; while the problem may try and trip you up by giving you the concentrations of other gases, these numbers are completely irrelevant.
Convert all the units in the problem to the correct units. For gases, the only major conversion needed is from degrees Celsius (C) to degrees Kelvin (K). You can convert from C to K by adding 273 degrees to the Celsius temperature. Therefore, if your gas is at room temperature, which is about 22 degrees C, you would add 273 to 22 to get 295 K.
Find the value of the universal gas constant, which is usually represented in chemistry and physics textbooks as R. This is a useful value to memorize, because you will need it quite often.
Multiply R by the temperature given to you in the problem. For fugacity, the universal gas constant is equal to 0.0821. Therefore, if you want to calculate the fugacity of a gas at room temperature, multiply 295 by 0.0821 to get 24.22.
Divide 1 by the number that you got in Step 4 to get the final value of the fugacity capacity in air. Note that the fugacity capacity is not the same as the fugacity. In this example, you would divide 1 by 24.22 to get 0.04.
Divide the concentration of your gas in air by the fugacity capacity that you calculated in the previous step. For a gas that exists in the air at a 2 Molar (M) concentration, you would divide 2 by 0.04 to get a final fugacity value of 50.