Volume of balloon = V = 800 cm3
Density of hydrogen gas = pH = 9 x 10-5 g cm-3
Weight of empty balloon = 0.3 gf
Density of air = ρa = 1.3 x 10-3 g cm-3
Weight of hydrogen gas in balloon = VρHg
= 800 x 9 x 10-5 x g
= 72 x 10-3 gf
= 0.072 gf
Volume of balloon = Volume of hydrogen gas in the balloon = volume of air displaced by balloon.
Down thrust = Wt. of empty balloon + wt. of hydrogen gas in balloon
= 0.3 + 0.072 = 0.372 gf
Upthrust = Wt. of air displaced by balloon
= Vρag
= 800 x 1.3 x 10-3 x g
= 8 x 1.3 x 10-1 x g
= 10.4 x 10-1 x g = 1.04 gf
Lifting power of balloon = Upthrust - Down thrust
= 1.04 - 0.372 = 0.668 gf