Using standard Chemistry notation and Mrzzys Molecular and atomic weights
CH4 =16
CO2=44
Air has an average Molecular mass based on percentage composition of approximately 28
(BTW Mrzzy if you are farting Ammonia you are in a LOT of trouble!! it should be excreted solubilised in urine)
H2S =34
Butyric Acid is 88
Now if you expel approx 25 Centimetres cubed of gas consisting of mostly Air and Co2 the net Molecular Mass will be about 28<35 so if we take the mean as 33

Acording to Avogadros theory 33grams(i mole) of the fart gas occupies 22.4 litres of volume
Calculation gives the mass of 25 mls. of fart gas as 0.036830357142857142857142857142857gms approximately