Question

How much mass is in 1 gallon of gasoline? If helium in a balloon at atmospheric P and T?

Answer 1

A volume of 1 gal equals 231 in³, the density is 46.8 lbm / ft³, 

so we get

m = pV = 46.8lbm / ft³ x 1 x (231 / 12³) ft³ = 6.256 lbm

A more accurate value is p = 848 lbm / ft³.

For the helium that density is 10.08 x 10^-3 lbm / ft³ 

so we get

m = pV = 10.08 x 10^-3 lbm / ft³ x 1 x (231 / 12³) ft³ = 0.00135 lbm