The is determined by utilizing a variation of the where density and molar mass replace moles and volume.
The original ideal gas law uses the formula PV = nRT, the density version of the ideal gas law is PM = dRT, where P is pressure measured in atmospheres (atm), T is temperature measured in kelvin (K), R is the ideal gas law constant 0.0821 ##(atm(L))/(mol(K))## just as in the original formula, but M is now the molar mass (##g/(mol)##) and d is the density (##g/L##).
By rearranging the formula to ##(PM)/(RT) = d## the units of atm, mol and K will cancel and the value will be left with the ##g/L## units for density.
