Gamma Question Bank for Engineering Chemistry>Chapter -1 - Principles of Qualitative Analysis>Assignment>Q 26

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Strontium sulphate is mixed with conc. $HCl$ and flame test is performed. The colour of flame is

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Important Points to Remember in Chapter -1 - Principles of Qualitative Analysis from Embibe Experts Gamma Question Bank for Engineering Chemistry Solutions

1. Charcoal Cavity Test:

Observation		Inference
Incrustation or Residue	Metallic bead
Yellow when hot, white when cold	None	${Z n}^{2 +}$
Brown when hot, yellow when cold	Grey bead which marks the paper	${P b}^{2 +}$
No characteristic residue	Red beads or scales	${C u}^{2 +}$
White residue which glows on heating	None	${B a}^{2 +}, {C a}^{2 +}, {M g}^{2 +}$
Black	None	Nothing definite-generally coloured salt

2. Cobalt Nitrate Test:

S. No.	Metal	Colour of the mass
1.	Zinc	Green
2.	Aluminium	Blue
3.	Magnesium	Pink
4.	Tin	Bluish – green

3. Flame Test:

Colour of Flame	Inference
Crimson Red / Carmine Red	Lithium
Golden yellow	Sodium
Violet / Lilac	Potassium
Brick red	Calcium
Apple Green / Yellowish Green	Barium
Green with a Blue centre/Greenish Blue	Copper

4. Borax Bead Test:

Metal	Colour in oxidising flame		Colour in reducing flame
Metal	When Hot	When Cold	When Hot	When Cold
Copper	Green	Blue	Colourless	Brownish red
Iron	Brownish yellow	Pale yellow / Yellow	Bottle green	Bottle green
Chromium	Yellow	Green	Green	Green
Cobalt	Blue	Blue	Blue	Blue
Manganese	Violet / Amethyst	Red / Amethyst	Grey/Colourless	Grey / Colourless
Nickel	Violet	Brown/Reddish brown	Grey	Grey

5. Analysis of ANIONS (Acidic Radicals):

(i) Dilute sulphuric acid/dilute hydrochloric acid group:

(a) Carbonate ion $({C O}_{3}^{2 -})$ :

Dilute $H_{2} {S O}_{4}$ test: A colourless odourless gas is evolved with brisk effervescence.

${C a C O}_{3} + H_{2} {S O}_{4} \to {C a S O}_{4} + H_{2} O + {C O}_{2} ↑$

Lime water/Baryta water $(B a (O H)_{2})$ test:

${C O}_{2} + C a (O H)_{2} ⟶ {C a C O}_{3} ↓ (m i l k y) + H_{2} O$

$C a C O_{3} + C O_{2} + H_{2} O \to C a {(H C O_{3})}_{2} (s o l u b l e) \overset{∆}{\to} C a C O_{3} ↓ + H_{2} O + C O_{2}$

(b) Sulphite ion $({S O}_{3}^{2 -})$ :

Dilute $H_{2} {S O}_{4}$ test:

${C a S O}_{3} + H_{2} {S O}_{4} ⟶ {C a S O}_{4} + H_{2} O + {S O}_{2} ↑$ ; ${S O}_{2}$ has suffocating odour of burning sulphur.

Acidified potassium dichromate test: The filter paper dipped in acidified $K_{2} {C r}_{2} O_{7}$ turns green.

${C r}_{2} O_{7}^{2 -} + 2 H^{+} + 3 {S O}_{2} ⟶ 2 {C r}^{3 +} (g r e e n) + 3 {S O}_{4}^{2 -} + H_{2} O$

Barium chloride/Strontium chloride solution:

${S O}_{3}^{2 -} + \frac{{B a}^{2 +}}{{S r}^{2 +}} ⟶ \frac{{B a S O}_{3}}{{S r S O}_{3}} ↓ (w h i t e)$

White precipitate dissolves in dilute $H C l$ .

${B a S O}_{3} ↓ + 2 H^{+} - {B a}^{2 +} + {S O}_{2} ↑ + H_{2} O$

(c) Sulphide ion $(S^{2 -})$ :

Dilute $H_{2} {S O}_{4}$ test: Pungent smelling gas like that of rotten egg is obtained.

$S^{2 -} + 2 H^{+} \to H_{2} S ↑$

Lead acetate test:

${({C H}_{3} C O O)}_{2} P b + H_{2} S \to P b S ↓ (b l a c k) + 2 {C H}_{3} C O O H$

Sodium nitroprusside test: Purple coloration is obtained.

$S^{2 -} + {[F e (C N)_{5} (N O)]}^{2 -} \to {[F e (C N)_{5} N O S]}^{4 -} (v i o l e t)$

Cadmium carbonate suspension/ Cadmium acetate solution:

${N a}_{2} S + {C d C O}_{3} \to C d S ↓ (Y e l l o w) + {N a}_{2} {C O}_{3}$

(d) Nitrite ion $({N O}_{2}^{-})$ :

Dilute $H_{2} {S O}_{4}$ test:

${N O}_{2}^{-} + H^{+} \to {H N O}_{2}; (2 {H N O}_{2} ⟶ H_{2} O + N_{2} O_{3})$

$3 {H N O}_{2} \to {H N O}_{3} + 2 N O + H_{2} O$ ; $2 N O + O_{2} ⟶ 2 {N O}_{2} ↑$

Starch iodide test:

$2 {N O}_{2}^{-} + 3 I^{-} + 4 {C H}_{3} C O O H \to I_{3}^{-} + 2 N O ↑ + 4 {C H}_{3} {C O O}^{-} + 2 H_{2} O$

$S t a r c h + I_{3}^{-} ⟶ B l u e$ (starch iodine adsorption complex)

(e) Acetate ion $({C H}_{3} {C O O}^{-})$

Dilute $H_{2} {S O}_{4}$ test:

${({C H}_{3} C O O)}_{2} C a + H_{2} {S O}_{4} ⟶ 2 {C H}_{3} C O O H (v i n e g a r l i k e s m e l l) + {C a S O}_{4}$

Neutral ferric chloride test:

$6 C H_{3} C O O^{-} + 3 F e^{3 +} + 2 H_{2} O \to {[F e_{3} (O H)_{2} {(C H_{3} C O O)}_{6}]}^{+} (d e e p red or b l o o d r e d c o l o u r a t i o n) + 2 H^{+}$

${[{F e}_{3} (O H)_{2} {({C H}_{3} C O O)}_{3}]}^{+} + 4 H_{2} O \to^{B o i l} 3 F e (O H)_{2} {C H}_{3} C O O ↓ (b r o w n i s h r e d) + 3 {C H}_{3} C O O H + H^{+}$

(ii) Conc. $H_{2} {S O}_{4}$ GROUP:

(a) Chloride ion $({C l}^{-})$ :

Concentrated $H_{2} {S O}_{4}$ test:

${C l}^{-} + H_{2} {S O}_{4} ⟶ H C l (c o l o u r l e s s p u n g e n t s m e l l i n g g a s) + {H S O}_{4}^{-}$

${N H}_{4} O H + H C l ⟶ {N H}_{4} C l ↑ (w h i t e f u m e s) + H_{2} O$

Silver nitrate test: ${C l}^{-} + {A g}^{+} ⟶ A g C l ↓ (w h i t e)$

White precipitate is soluble in aqueous ammonia and precipitate reappears with ${H N O}_{3}$ .

$A g C l + 2 {N H}_{4} O H ⟶ [A g {({N H}_{3})}_{2}] C l (S o l u b l e) + 2 H_{2} O$ ;

$[A g {({N H}_{3})}_{2}] C l + 2 H^{+} ⟶ A g C l ↓ + 2 {N H}_{4}^{+}$

Chromyl chloride test:

$4 {C l}^{-} + {C r}_{2} O_{7}^{2 -} + 6 H^{+} (c o n c .) \to 2 {C r O}_{2} {C l}_{2} (d e e p r e d v a p o u r s) + 3 H_{2} O$

$C r O_{2} C l_{2} + 4 O H^{-} \to C r O_{4}^{2 -} + 2 C l^{-} + 2 H_{2} O$ ;

${C r O}_{4}^{2 -} + {P b}^{+ 2} \to {P b C r O}_{4} ↓ (y e l l o w)$

(b) Bromide ion $({B r}^{-})$ :

Concentrated $H_{2} {S O}_{4}$ test:

$2 N a B r + H_{2} {S O}_{4} \to {N a}_{2} {S O}_{4} + 2 H B r$ ;

$2 H B r + H_{2} {S O}_{4} \to {B r}_{2} ↑ (r e d d i s h b r o w n) + 2 H_{2} O + {S O}_{2}$

Silver nitrate test:

$N a B r + {A g N O}_{3} \to A g B r ↓ (p a l e y e l l o w) + {N a N O}_{3}$

Yellow precipitate is partially soluble in dilute aqueous ammonia but readily dissolves in concentrated ammonia solution.

$A g B r + 2 {N H}_{4} O H \to [A g {({N H}_{3})}_{2}] B r + H_{2} O$

Chlorine water test (organic layer test):

$2 {B r}^{-} + {C l}_{2} \to 2 {C l}^{-} + {B r}_{2}$

$B r_{2} + \frac{C H C l_{3}}{C C l_{4}} ⟶ B r_{2}$ dissolve to give reddish brown colour in organic layer.

(c) Iodide ion $(I^{-})$ :

Concentrated $H_{2} {S O}_{4}$ test:

$2 N a I + H_{2} {S O}_{4} \to {N a}_{2} {S O}_{4} + 2 H I$

$2 H I + H_{2} {S O}_{4} \to I_{2} ↑$ (pungent smelling dark violet) $+ 2 H_{2} O + {S O}_{2}$

Starch paper test:

Iodides are readily oxidised in acid solution to free iodine; the free iodine may than be identified by deep blue colouration produced with starch solution.

$3 I^{-} + 2 {N O}_{2}^{-} + 4 H^{+} ⟶ I_{3}^{-} + 2 N O ↑ + 2 H_{2} O$ .

Silver nitrate test:

Bright yellow precipitate is formed.

$I^{-} + {A g}^{+} \to A g I ↑$

Bright yellow precipitate is insoluble in dilute aqueous ammonia but is partially soluble in concentrated ammonia solution.

Chlorine water test (organic layer test):

$2 N a I + {C l}_{2} \to 2 N a C l + I_{2}$

$I_{2} + {C H C l}_{3} ⟶ I_{2}$ dissolves to give violet colour in organic layer.

(d) Nitrate ion $({N O}_{3}^{-}) :$

Concentrated $H_{2} {S O}_{4}$ test:

Pungent smelling reddish-brown vapours are evolved.

$4 {N O}_{3}^{-} + 2 H_{2} {S O}_{4} \to 4 {N O}_{2} ↑ + O_{2} + 2 {S O}_{4}^{2 -} + 2 H_{2} O$

Addition of bright copper turnings or paper pellets intensifies the evolution of reddish-brown gas.

$2 {N O}_{3}^{-} + 4 H_{2} {S O}_{4} + 3 C u \to 3 {C u}^{2 +} + 2 N O ↑ + 4 {S O}_{4}^{2 -} + 4 H_{2} O_{2}$ ; $2 N O ↑ + O_{2} ⟶ 2 {N O}_{2} ↑$

$4 C (p a p e r p e l l e t) + 4 {H N O}_{3} \to 2 H_{2} O + 4 {N O}_{2} + 4 {C O}_{2}$ .

Brown ring test:

$2 {N O}_{3}^{-} + 4 H_{2} {S O}_{4} + 6 {F e}^{2 +} \to 6 {F e}^{3 +} + 2 N O ↓ + 4 S O_{4}^{2 -} + 4 H_{2} O$ .

${F e}^{2 +} + N O ↑ + 5 H_{2} O \to {[{F e}^{I} {(H_{2} O)}_{5} {N O}^{+}]}^{2 +} (b r o w n r i n g)$ .

(iii) Miscellaneous Group:

(a) Sulphate ion $({S O}_{4}^{2 -})$ :

Barium chloride test:

${N a}_{2} {S O}_{4} + {B a C l}_{2} ⟶ {B a S O}_{4} ↓ (w h i t e) + 2 N a C l$ .

White precipitate is insoluble in warm dil. ${H N O}_{3}$ as well as $H C l$ but moderately soluble in boiling concentrated hydrochloric acid.

Lead acetate test:

${N a}_{2} {S O}_{4} + {({C H}_{3} C O O)}_{2} P b ⟶ {P b S O}_{4} ↓ (W h i t e) + 2 {C H}_{3} C O O N a$

White precipitate soluble in excess of hot ammonium acetate

${P b S O}_{4} + 2 {C H}_{3} {C O O N H}_{4} \to {({C H}_{3} C O O)}_{2} P b (s o l u b l e) + {({N H}_{4})}_{2} {S O}_{4}$

(b) Phosphate ion $({P O}_{4}^{3 -})$ :

Ammonium molybdate test:

${N a}_{2} {H P O}_{4} (a q) + 12 {({N H}_{4})}_{2} {M o O}_{4} + 23 {H N O}_{3} \to {({N H}_{4})}_{3} {P M o}_{12} O_{40} ↓ (c a n a r y y e l l o w) + 2 {N a N O}_{3} + 21 {N H}_{4} {N O}_{3} + 12 H_{2} O$

6. Analysis of cations:

Group 0: Ammonium ion $({N H}_{4}^{+})$ :

$2 {N H}_{3} + {M n}^{2 +} + H_{2} O_{2} + H_{2} O \to M n O (O H)_{2} ↓ (b r o w n) + 2 {N H}_{4}^{+}$

Nessler's reagent test (Alkaline solution of potassium tetraidomercurate $(l l)$ :

${N H}_{4}^{+} + 2 {[{H g I}_{4}]}^{2 -} + 4 {O H}^{-} \to H g O H g ({N H}_{2}) I ↓ (b r o w n) + 7 I^{-} + 3 H_{2} O$

$3 {N H}_{4}^{+} + {[C o {({N O}_{2})}_{6}]}^{3 -} \to ({N H}_{4}) [C o {({N O}_{2})}_{6}] ↓ (y e l l o w)$

$2 {N H}_{4}^{+} + {[{P t C l}_{6}]}^{-} \to {({N H}_{4})}_{2} [{P t C l}_{6}] ↓ y e l l o w$

7. $1^{s t}$ GROUP $({P b}^{2 +}, {H g}_{2}^{2 +}, {A g}^{+})$ :

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8. $I I A$ GROUP

$({H g}^{2 +}, {P b}^{2 +}, {B i}^{3 +}, {C u}^{2 +}, {C d}^{2 +})$ :

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9. $I I B$ GROUP $(A s^{3 +}, {S b}^{3 +}, {S n}^{2 +}, {S n}^{4 +})$ :

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10. $I I I^{r d}$ GROUP $({A l}^{+ 3}, {C r}^{+ 3}, F e^{+ 3})$ :

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11. $I V^{t h}$ GROUP $({Z n}^{2 +}, {M n}^{2 +}, {N i}^{2 +}, {C o}^{2 +})$ :

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12. $V^{t h}$ GROUP $({B a}^{2 +}, {S r}^{2 +}, {C a}^{2 +})$ :

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13. $V I^{t h}$ GROUP:

(i) MAGNESIUM ION $({M g}^{2 +})$ :

${M g}^{2 +} + {N H}_{3} + {H P O}_{4}^{2 -} ⟶ M g ({N H}_{4}) {P O}_{4} ↓ (w h i t e)$

$5 {M g}^{2 +} + 6 {C O}_{3}^{2 -} + 7 H_{2} O \to 2 {M g C O}_{3} \cdot M g (O H)_{2} \cdot 5 H_{2} O ↓ + 2 {H C O}_{3}^{-}$

(ii) Titan Yellow (a water-soluble yellow dyestuff): It is adsorbed by $M g (O H)_{2}$ producing a deep red colour or precipitate.