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1.) A silver coin which weights 26.85g is dissolved in 300 cm³ of nitric acid. 30 cm³ of the resulting solution requires 36.3 cm³ of 0.6 M HCl for complete precipitation .Find the percentage purity of the coin.

Consider the precipitation:
AgNO3 + HCl → AgCl↓ + HNO3
Mole ratio AgNO3 : HCl = 1 : 1
No. of moles of HCl = 0.6 x (36.3/1000) = 0.02178 molNo. of moles of AgNO3 in the diluted solution = 0.02178 mol
No. of moles of AgNO3 in the original solution = 0.02178 x (300/30) = 0.2178 mol

1 mol of Ag gives 1 mol of AgNO3.
No. of moles of Ag in the coin = 0.2178 mol
Mass of Ag in the coin = 0.2178 x 107.9 = 23.50 g
% purity = (23.50/26.85) x 100% = 87.52%


2.) Find the mass of oxygen required to produce 20 g of water.
2H2 + O2 → 2H2O
Mole ratio O2 : H2O = 1 : 2

Molar mass of H2O = 1x2 + 16 = 18 g/mol
Mass fraction of O in H2O = 16/18
Molar mass of O2 = 16x2 = 32 g/mol

No. of moles of H2O produced = 20/18 mol
No. of moles of O2 required = (20/18) x (1/2) = 5/9 mol
Mass of O2 required = (5/9) x 32 = 17.8 g (3 sig. fig.)