Vidaara.orgClass 11 · Chemistry
CodeVID-C11-12-T3-01
Assignment — Purification & Analysis
Name: ____________________
Roll No.: __________
Date: ____________
General Instructions
- All questions are compulsory.
- Section A carries 1 mark each, Section B 2 marks, Section C 3 marks and Section D 5 marks.
- Show all working for Sections B, C and D. Only final answers are given at the end — for full solutions, raise your doubts with your teacher.
Section A — Multiple Choice Questions
5 × 1 = 5 marks
1.
Two liquids with very close boiling points are best separated by:
- A.simple distillation
- B.fractional distillation
- C.sublimation
- D.crystallisation
2.
Hydrogen in an organic compound is detected as water, which turns:
- A.lime-water milky
- B.anhydrous $CuSO_4$ blue
- C.starch blue
- D.litmus red
3.
Sulphur in Lassaigne's extract gives a violet colour with:
- A.$AgNO_3$
- B.$FeCl_3$
- C.sodium nitroprusside
- D.lime-water
4.
Halogens are estimated as silver halide in the:
- A.Dumas method
- B.Kjeldahl method
- C.Carius method
- D.Liebig method
5.
Aniline, immiscible with water and steam-volatile, is purified by:
- A.steam distillation
- B.reduced-pressure distillation
- C.sublimation
- D.chromatography
Section B — Short Answer (2 marks)
3 × 2 = 6 marks
6.
State the principle of crystallisation as a purification method.
7.
Why is sodium fusion (Lassaigne's test) required before testing for nitrogen, sulphur or halogens?
8.
Name two compounds purified by sublimation and the property exploited.
Section C — Short Answer (3 marks)
2 × 3 = 6 marks
9.
On combustion, $0.45\ \text{g}$ of an organic compound gave $0.66\ \text{g}$ $CO_2$ and $0.27\ \text{g}$ $H_2O$. Find $\%C$ and $\%H$.
10.
Compare the Dumas and Kjeldahl methods for estimating nitrogen.
Section D — Long Answer (5 marks)
1 × 5 = 5 marks
11.
Describe the Kjeldahl method for estimating nitrogen, with the chemical changes and the percentage formula; then compute $\%N$ if $0.40\ \text{g}$ of compound needs $20\ \text{mL}$ of $0.5\ \text{N}$ acid to neutralise the liberated ammonia.
Answer Key
Section A — Multiple Choice Questions
- (B) fractional distillation
- (B) anhydrous $CuSO_4$ blue
- (C) sodium nitroprusside
- (C) Carius method
- (A) steam distillation
Section B — Short Answer (2 marks)
- The compound is dissolved in a hot solvent in which it is much less soluble when cold; on cooling, pure crystals separate while soluble impurities remain dissolved in the mother liquor.
- These elements are covalently bonded and not directly ionisable. Fusion with sodium converts them into ionic compounds ($NaCN$, $Na_2S$, $NaX$) that can be detected by simple ionic tests.
- Naphthalene and camphor (also benzoic acid) are purified by sublimation, which exploits their ability to change directly from solid to vapour, leaving non-sublimable impurities behind.
Section C — Short Answer (3 marks)
- $\%C=\dfrac{12}{44}\times\dfrac{0.66}{0.45}\times100=40\%$; $\%H=\dfrac{2}{18}\times\dfrac{0.27}{0.45}\times100=6.67\%$.
- Dumas oxidises nitrogen to $N_2$ gas (measured over KOH) and works for all N compounds. Kjeldahl converts N to $(NH_4)_2SO_4$, liberates $NH_3$ and titrates it ($\%N=\dfrac{1.4\,N_{acid}V_{acid}}{m}$) but fails for nitro, azo and ring nitrogen.
Section D — Long Answer (5 marks)
- The compound is heated with conc. $H_2SO_4$ and a catalyst ($K_2SO_4$ + $CuSO_4$) so that nitrogen is converted to $(NH_4)_2SO_4$. Excess NaOH then liberates ammonia, which is distilled into a known excess of standard acid; the unreacted acid is back-titrated, giving the acid that reacted with $NH_3$. Then $\%N=\dfrac{1.4\times N_{acid}\times V_{acid}}{m}$. Substituting: $\%N=\dfrac{1.4\times 0.5\times 20}{0.40}=\dfrac{14}{0.40}=35\%$. The method fails for nitro, azo and ring (pyridine) nitrogen, where Dumas is used instead.
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