How many pounds of air in a 3 gallon tank?

3 US gallons = 13,63827 L
Standardized air density: 1,225 g/L
Mass of air: 13,63827 x 1,225 = 16,70688075 g
The equivalent of of 16,707 g in pounds is 0,037.
It is strongly recommended to use only SI units !