Magnetite can be reduced with CO to yield iron metal and carbon dioxide. Calculate the mass of magnetite (in kg ) needed to obtain 4 kg of iron if the process is $80 \%$ efficient. [Atomic weight of Fe and O are 56 g and 16 g , respectively]

Combustion of 10 mL of a gaseous hydrocarbon gives 40 mL of $\mathrm{CO}_2$ and 50 mL of water vapour under the same conditions. The molecular formula of the hydrocarbon is

The average molecular weight of the air which has $21 \%$ of $\mathrm{O}_2$ and $79 \%$ of $\mathrm{N}_2$, is

The number of significant figures in 2.0400 is