Q4. Explain the difference between the principles of the Dumas and Kjeldahl methods for nitrogen (NCERT Exercise 8.22).
Dumas: the compound is heated with CuO in CO2, and its nitrogen is set free as N2, whose volume is measured (after absorbing CO2 in KOH). Kjeldahl: the compound is digested with concentrated H2SO4, which turns nitrogen into ammonium sulphate; the ammonia liberated with NaOH is absorbed in standard acid and estimated by titration. Dumas has broader applicability, provided combustion and reduction are complete; the unmodified textbook Kjeldahl procedure excludes nitro, azo and pyridine-like ring nitrogen.
Q5. Why is potassium hydroxide solution used to absorb the CO2 in the estimation of carbon (NCERT Exercise 8.30)?
CO2 is an acidic oxide and reacts completely with the base: 2KOH + CO2 → K2CO3 + H2O. The gain in mass of the KOH tube is therefore the mass of CO2. The CaCl2 tube comes first so that KOH solution does not also take up water from the gas.
Q6. In a Dumas estimation, 0.25 g of a compound gave 31.7 mL of moist nitrogen at 290 K and 750 mm. The aqueous tension at 290 K is 14.5 mm. Find the percentage of nitrogen.
Pressure of N2 = 750 − 14.5 = 735.5 mm. Volume at STP = (735.5 × 31.7 × 273)/
Q7. 0.20 g of a compound gave 0.111 g of Mg2P2O7. Find the percentage of phosphorus.
% P = (62 × 0.111 × 100)/
Q8. In the direct estimation of oxygen, 0.30 g of a compound gave 0.264 g of CO2 after the iodine pentoxide step. Find the percentage of oxygen.
Each 32 g of oxygen gives 88 g of CO2. % O = (32 × 0.264 × 100)/
Q9. State the principle of the estimation of halogens, sulphur and phosphorus (NCERT Exercise 8.23).
In each, the compound is oxidised so the element becomes a simple ion that is precipitated and weighed. Halogens: heated with fuming HNO3 and AgNO3 in a Carius tube, weighed as AgX. Sulphur: oxidised to H2SO4 and weighed as BaSO4. Phosphorus: oxidised to H3PO4 and weighed as ammonium phosphomolybdate or as Mg2P2O7.