An engineer wants to estimate the mass of gas that is present in a tank

Well, in order to obtain the mass of the gas in grams, we have to find out the number of moles. Since the eqn is Moles = Mass(g)/ar or mr and if we reconfigure the eqn to Mass= moles × ar or mr. To find out moles, we have to derive it from the concentration of it since concentration= Mol/dm3 amd if we reconfigure the eqn to moles= concentration × dm3, we can obtain the numner of moles. After that, imput the number of moles into the eqn: mass= moles × ar or mr. Tada.

Oh yeah, you must know what is the gas in the tank. If not, it is impossible.
Ar means relative atomic mass
Mr means relative molecular mass.

Convection is da answer my dood