An oxide of iron contains $69.9 \%$ iron, its empirical formula, is :

(Given : Molar mass of Fe and O are 56 and $16 \mathrm{~g} \mathrm{~mol}^{-1}$ respectively.)

What volume of hydrogen gas at STP would be liberated by action of 50 mL of $\mathrm{H}_2 \mathrm{SO}_4$ of $50 \%$ purity (density $=1.3 \mathrm{~g} \mathrm{~mL}^{-1}$ ) on 20 g of zinc?

Given : Molar mass of $\mathrm{H}, \mathrm{O}, \mathrm{S}, \mathrm{Zn}$ are 1, 16, 32, $65 \mathrm{~g} \mathrm{~mol}^{-1}$ respectively.

How many grams of residue is obtained by heating 2.76 g of silver carbonate?

(Given : Molar mass of $\mathrm{C}, \mathrm{O}$ and Ag are 12, 16 and $108 \mathrm{~g} \mathrm{~mol}^{-1}$ respectively)

What is the mole fraction of water in $10 \%$ by weight $(\mathrm{w} / \mathrm{w})$ of aqueous urea solution?

[Given: Molar mass of $\mathrm{H}, \mathrm{O}, \mathrm{C}$ and N are $1,16,12$ and $14 \mathrm{~g} \mathrm{~mol}^{-1}$ respectively.]