Volume of balloon = V = 800 cm^{3}

Density of hydrogen gas = p_{H} = 9 x 10^{-5} g cm^{-3}

Weight of empty balloon = 0.3 gf

Density of air = ρ_{a} = 1.3 x 10^{-3} g cm^{-3}

**Weight of hydrogen gas in balloon = Vρ**_{H}g

= 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

= Vρ_{a}g

= 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**