More practice questions

3. Q3. Lime water turns milky with both CO₂ and SO₂. How does the preliminary test tell them apart?

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   CO₂ is odourless, while SO₂ smells of burning sulphur; SO₂ also turns acidified potassium dichromate green (Lab Manual, pages 51–52).

4. Q4. Why does lime water go milky with CO₂ and then clear again?

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   CO₂ first precipitates calcium carbonate, CaCO₃; more CO₂ turns it into calcium hydrogencarbonate, Ca(HCO₃)₂, which dissolves in water (Lab Manual, page 53).

5. Q5. What colour does the sodium carbonate extract of a sulphide give with sodium nitroprusside, and what forms?

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   A purple or violet colour, from the complex Na₄[Fe(CN)₅NOS]: Na₂S + Na₂[Fe(CN)₅NO] → Na₄[Fe(CN)₅NOS] (Lab Manual, page 54).

6. Q6. A solution containing 0.585 g of NaCl is treated with excess silver nitrate. What mass of AgCl precipitates? (Na = 23, Cl = 35.5, Ag = 108.)

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   M(NaCl) = 58.5 g mol⁻¹, so 0.585 g is 0.0100 mol, giving 0.0100 mol of AgCl, M = 108 + 35.5 = 143.5 g mol⁻¹: 1.435 g, about 1.44 g (reasoned from the Lab Manual’s equation, page 58).