Adyasha,

Consider a pure gas of ammonia. It has, at room temperature, a mass of 17 grammes per 24 litres of volume.

If I have one litre of this gas it has a mass of ~0.29 grammes.

If I have a gas mixture that contains 1cc of that pure gas in 999 cc of some other gas I have a 1:1000 mixture, right?

And the amount of ammonia in that mix is 0.29/1000 grammes.

No formula as such, just be aware of what you're looking for and keep in mind the molar volume of all gases at rtp or stp.