Analytical Chemistry

What do you understand by the following:

(a) Analysis

(b) Qualitative analysis

(c) Reagent

(d) Precipitation

Answer

(a) Analysis — Determination of the chemical components in a given sample is called Analysis.

(b) Qualitative analysis — Identification of the unknown substances in a given sample is called Qualitative analysis.

(c) Reagent — A reagent is a substance that reacts with another substances.

(d) Precipitation — The process of formation of an insoluble solid when solutions are mixed is called Precipitation. The solid thus formed is called Precipitate.

Write the probable colour of the following salts:

(a) Iron (III) chloride

(b) Potassium nitrate

(c) Ferrous sulphate

(d) Aluminium acetate

Answer

(a) Iron (III) chloride — Yellow

(b) Potassium nitrate — Colourless

(c) Ferrous sulphate — Pale Green

(d) Aluminium acetate — Colourless

Name the probable cation present based on the following observations:

(a) White precipitate insoluble in NH₄OH but soluble in NaOH.

(b) Blue coloured solution.

Answer

(a) Pb²⁺

(b) Cu²⁺

Name the metal hydroxides which are:

(a) Insoluble

(b) Soluble

in

(i) caustic soda solution

(ii) Ammonium hydroxide solution

Answer

What do you observe when ammonium salt is heated with caustic soda solution? Write the word equation:

Answer

When ammonium salt is heated with caustic soda solution, ammonia gas is evolved.

The word equation is:

Ammonium Salt + Sodium Hydroxide $\xrightarrow{\space\Delta\space}$ Sodium Salt + Water + Ammonia Gas

How will you distinguish NH₄OH solution from NaOH solution?

Answer

NH₄OH and NaOH can be distinguished by using CuSO₄.

CuSO₄ forms a pale blue precipitate which is insoluble in excess of sodium hydroxide and with ammonium hydroxide it forms a pale blue precipitate which dissolves in excess of ammonium hydroxide and forms a deep/inky blue solution.

$\underset{\text{blue}}{{\text{CuSO}_4}} + \underset{\text{caustic soda - colourless}}{2\text{NaOH}} \longrightarrow \underset{\text{pale blue ppt}}{\text{Cu(OH)}_2} ↓ + \underset{\text{colourless}}{\text{Na}_2\text{SO}_4}$

$\underset{\text{blue}}{{\text{CuSO}_4}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{pale blue ppt}}{\text{Cu(OH)}_2} ↓ + \underset{\text{colourless in solution}}{(\text{NH}_4)_2\text{SO}_4}$

$\text{Cu(OH)}_2 + (\text{NH}_4)_2\text{SO}_4 + 2\text{NH}_4\text{OH} \longrightarrow \underset{\underset{\text{copper (II) sulphate}}{\text{Tetraammine}}}{[(\text{Cu(NH}_3)_4]\text{SO}_4} + 4\text{H}_2\text{O}$

Why the alkali is added drop by drop to the salt solution?

Answer

If an alkali is added too quickly, then it is easy to miss a precipitate that redissolves in excess alkali.

Write balanced equation:

(a) Reaction of sodium hydroxide solution with Iron (III) chloride solution

(b) Copper sulphate solution with ammonium hydroxide solution

Answer

(a) When sodium hydroxide solution is added to FeCl₃ dropwise, a reddish brown ppt is obtained, which is insoluble in excess of NaOH:

$\underset{\text{Yellow}}{{\text{FeCl}_3}} + \underset{\text{Colourless}}{3\text{NaOH}} \longrightarrow \underset{\text{Reddish brown ppt}}{\text{Fe(OH)}_3↓} + \underset{\text{Colourless}}{3\text{NaCl}}$

(b) When ammonia solution is added dropwise to cupper sulphate, a pale blue ppt of copper hydroxide is obtained.

$\underset{\text{Blue}}{{\text{CuSO}_4}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{Pale Blue ppt}}{\text{Cu(OH)}_2} ↓ + \underset{\text{Colourless in solution}}{(\text{NH}_4)_2\text{SO}_4}$

On adding excess of ammonia solution, the ppt dissolves and a deep blue solution is obtained.

$\text{Cu(OH)}_2 + (\text{NH}_4)_2\text{SO}_4 + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{Tetraammine copper (II) sulphate}}{[(\text{Cu(NH}_3)_4]\text{SO}_4} + 4\text{H}_2\text{O}$

The colour of an aqueous solution of copper sulphate is:

1. Green
2. Brown
3. Blue
4. Yellow

Answer

Blue

Reason — Aqueous solution of copper sulphate is blue.

The colour of the precipitate formed on adding NaOH solution to iron (II) sulphate solution is:

1. White
2. Brown
3. Green
4. Pale blue

Answer

Green

Reason — Dirty green precipitate of Ferrous Hydroxide [Fe(OH)₂] is formed.

A metal which produces hydrogen on reacting with alkali as well as with acid:

1. Iron
2. Magnesium
3. Zinc
4. Copper

Answer

Zinc

Reason — Reactions of Zinc with alkali and acid are shown below:
Zn + 2NaOH ⟶ Na₂(ZnO)₂ + H₂↑
Zn + HCl ⟶ ZnCl₂ + H₂↑

The salt solution which does not react with ammonium hydroxide is:

1. Calcium nitrate
2. Zinc nitrate
3. Lead nitrate
4. Copper nitrate

Answer

Calcium Nitrate

Reason — No ppt. occurs even with addition of excess of ammonium hydroxide as the concentration of OH^- ions from the ionization of of NH₄OH is so low that it cannot precipitate the hydroxide of calcium.

Name:

(a) a metallic hydroxide soluble in excess of NH₄OH.

(b) a metallic oxide soluble in excess of caustic soda solution.

(c) a strong alkali.

(d) a weak alkali.

(e) two colourless metal ions.

(f) two coloured metal ions.

(g) a metal that evolves a gas which burns with a pop sound when boiled with alkali solutions.

(h) two bases which are not alkalis but dissolve in strong alkalis.

(i) a coloured metallic oxide which dissolves in alkalis to yield colourless solutions.

(j) a colourless cation not a representative element.

(k) a yellow monoxide that dissolves in hot and concentrated caustic alkali.

(l) a white, insoluble oxide that dissolves when fused with caustic soda or caustic potash.

(m) a compound containing zinc in the anion.

Answer

(a) Copper hydroxide [Cu(OH)₂]

(b) Zinc oxide [ZnO]

(c) Sodium hydroxide [NaOH]

(d) Ammonium hydroxide [NH₄OH]

(e) Sodium ion [Na⁺] , Potassium ion [K⁺]

(f) Cupric ion [Cu²⁺], Ferrous ion [Fe²⁺]

(g) Aluminium [Al]

(h) Zinc hydroxide [Zn(OH)₂] and Lead hydroxide [Pb(OH)₂]

(i) Lead oxide [PbO]

(j) Ammonium ion [NH₄⁺]

(k) Lead oxide [PbO]

(l) Zinc oxide [ZnO]

(m) Potassium Zincate [K₂ZnO₂]

Write the probable colour of the following salts?

(a) Ferrous salts

(b) Ammonium salts

(c) Cupric salts

(d) Calcium salts

(e) Aluminium salts

Answer

(a) Ferrous salts — Pale Green

(b) Ammonium salts — Colourless

(c) Cupric salts — Blue

(d) Calcium salts — Colourless

(e) Aluminium salts — Colourless

Name the chloride of a metal which is soluble in excess of ammonium hydroxide. Write equation for the same.

Answer

Zinc chloride (ZnCl₂) is soluble in excess of ammonium hydroxide.

When ammonia solution is added dropwise to zinc chloride solution, a white gelatinous ppt of zinc hydroxide is obtained.

$\underset{\text{colourless solution}}{{\text{ZnCl}_2}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white gelatinous ppt}}{\text{Zn(OH)}_2↓} + 2\text{NH}_4\text{Cl}$

On adding excess of ammonia solution, the ppt dissolves and a colourless solution is obtained.

$\text{Zn(OH)}_2 + 2\text{NH}_4\text{Cl} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\underset{\text{zinc (II) sulphate}}{\text{Tetraammine}}}{[(\text{Zn(NH}_3)_4]\text{Cl}_2} + 4\text{H}_2\text{O}$

What happens when ammonia solution is added first dropwise and then in excess to the following solutions:

(i) CuSO₄

(ii) ZnSO₄

(iii) FeCl₃

Write balanced equations for these reactions.

Answer

(i) When ammonia solution is added dropwise to cupper sulphate, a pale blue ppt of copper hydroxide is obtained.

$\underset{\text{blue}}{{\text{CuSO}_4}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{pale blue ppt}}{\text{Cu(OH)}_2} ↓ + \underset{\text{colourless in solution}}{(\text{NH}_4)_2\text{SO}_4}$

On adding excess of ammonia solution, the ppt dissolves and a deep blue solution is obtained.

$\text{Cu(OH)}_2 + (\text{NH}_4)_2\text{SO}_4 + 2\text{NH}_4\text{OH} \longrightarrow \underset{\underset{\text{copper (II) sulphate}}{\text{Tetraammine}}}{[(\text{Cu(NH}_3)_4]\text{SO}_4} + 4\text{H}_2\text{O}$

(ii) When ammonia solution is added dropwise to zinc sulphate, a white gelatinous ppt of zinc hydroxide is obtained.

$\underset{\text{colourless solution}}{{\text{ZnSO}_4}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white gelatinous ppt}}{\text{Zn(OH)}_2↓} + \underset{\text{colourless in solution}}{(\text{NH}_4)_2\text{SO}_4}$

On adding excess of ammonia solution, the ppt dissolves and a colourless solution is obtained.

$\text{Zn(OH)}_2 + (\text{NH}_4)_2\text{SO}_4 + 2\text{NH}_4\text{OH} \longrightarrow \underset{\underset{\text{zinc (II) sulphate}}{\text{Tetraammine}}}{[(\text{Zn(NH}_3)_4]\text{SO}_4} + 4\text{H}_2\text{O}$

(iii) When ammonia solution is added dropwise to iron (III) chloride, a reddish brown ppt. of Fe(OH)₃ is obtained.

$\underset{\text{yellow solution}}{{\text{FeCl}_3}} + 3\text{NH}_4\text{OH} \longrightarrow \underset{\text{reddish brown ppt}}{\text{Fe(OH)}_3↓} + \underset{\text{colourless in solution}}{3\text{NH}_4\text{Cl}}$

Excess of ammonia solution addition doesn't dissolve ppt.

What do you observe when caustic soda solution is added to the following solution, first a little and then in excess :

(a) FeCl₃

(b) ZnSO₄

(c) Pb(NO₃)₂

(d) CuSO₄

Write balanced equations for these reactions.

Answer

(a) When caustic soda solution is added to FeCl₃ dropwise, a reddish brown ppt is obtained, which is insoluble in excess of NaOH:

$\underset{\text{yellow}}{{\text{FeCl}_3}} + \underset{\text{colourless}}{3\text{NaOH}} \longrightarrow \underset{\text{reddish brown ppt}}{\text{Fe(OH)}_3↓} + \underset{\text{colourless}}{3\text{NaCl}}$

(b) When caustic soda solution is added to Zinc sulphate dropwise, a white gelatinous ppt is obtained, which dissolves in excess of NaOH:

$\underset{\text{colourless}}{{\text{ZnSO}_4}} + \underset{\text{colourless}}{2\text{NaOH}} \longrightarrow \underset{\text{white gelatinous ppt}}{\text{Zn(OH)}_2↓} + \underset{\text{ colourless}}{\text{Na}_2\text{SO}_4}$

${\text{Zn(OH)}_2} + \underset{\text{ excess}}{2\text{NaOH}} \longrightarrow \underset{\text{colourless}}{\text{Na}_2\text{ZnO}_2↓} + 2\text{H}_2\text{O}$

(c) When caustic soda solution is added to Pb(NO₃)₂ dropwise, a chalky white ppt is obtained, which dissolves in excess of NaOH:

$\underset{\text{colourless}}{{\text{Pb(NO}_3)_2}} + \underset{\text{colourless}}{2\text{NaOH}} \longrightarrow \underset{\text{white ppt}}{\text{Pb(OH)}_2↓} + \underset{\text{ colourless}}{2\text{NaNO}_3}$

${\text{Pb(OH)}_2} + \underset{\text{excess}}{2\text{NaOH}} \longrightarrow \underset{\text{sodium plumbite - colourless}}{\text{Na}_2\text{PbO}_2↓} + 2\text{H}_2\text{O}$

(d) When caustic soda solution is added to CuSO₄ dropwise, a pale blue ppt is obtained, which is insoluble in excess of NaOH:

$\underset{\text{blue}}{{\text{CuSO}_4}} + \underset{\text{colourless}}{2\text{NaOH}} \longrightarrow \underset{\text{pale blue ppt}}{\text{Cu(OH)}_2↓} + \underset{\text{colourless}}{\text{Na}_2\text{SO}_4}$

What do you observe when freshly precipitated aluminum hydroxide reacts with caustic soda solution? Give balanced equation.

Answer

When freshly precipitated aluminum hydroxide reacts with caustic soda solution, a white soluble salt of sodium meta aluminate is obtained.

Al(OH)₃ + NaOH ⟶ NaAlO₂ [soluble] + 2H₂O

What is observed when hot concentrated caustic soda solution is added to

(a) Zinc

(b) Aluminium?

Write balanced equations.

Answer

(a) When hot concentrated caustic soda solution is added to zinc, soluble salt of sodium zincate [Na₂ZnO₂] is formed and hydrogen gas is liberated.

The balanced equation is:

$\text{Zn} + \underset{\text{hot and conc.}}{2\text{NaOH}} \longrightarrow \underset{\underset{\text{colourless}}{\text{sodium zincate}}}{\text{Na}_2\text{ZnO}_2} + \text{H}_2↑$

(b) When hot concentrated caustic soda solution is added to aluminium, soluble salt of sodium meta aluminate [NaAlO₂] is formed and hydrogen gas is liberated.

$2\text{Al} + 2\text{NaOH} + 2\text{H}_2\text{O} \longrightarrow \underset{\underset{\text{colourless}}{\text{sodium meta aluminate}}}{2\text{NaAlO}_2} + 3\text{H}_2↑$

Distinguish by adding: Sodium hydroxide solution or Ammonium hydroxide solution to

(a) Calcium salt solution and lead salt solution

(b) Lead nitrate solution and zinc nitrate solution

(c) Copper salt solution and ferrous salt solution

(d) Fe(II) salt solution and Fe(III) salt solution

(e) Ferrous nitrate and lead nitrate

Answer

(a) Ammonium hydroxide on reaction with lead salt solution gives chalky white precipitate of Pb(OH)₂. No precipitation occurs on adding Ammonium hydroxide to Calcium salt solution even when it is added in excess.

$\underset{\text{colourless}}{{\text{Pb(NO}_3)_2}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white ppt}}{\text{Pb(OH)}_2} ↓ + 2\text{NH}_4\text{NO}_3$

(b) When ammonium hydroxide solution is added to each of the compounds, lead nitrate forms a chalky white precipitate of lead hydroxide [Pb(OH)₂] which is insoluble in excess of ammonium hydroxide.

$\underset{\text{colourless}}{{\text{Pb(NO}_3)_2}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white ppt}}{\text{Pb(OH)}_2} ↓ + 2\text{NH}_4\text{NO}_3$

Whereas a gelatinous white precipitate of zinc hydroxide [Zn(OH)₂] is formed in case of zinc nitrate, which is soluble in excess of ammonium hydroxide.

Zn(NO₃)₂ + 2NH₄OH ⟶ 2NH₄NO₃ + Zn(OH)₂ ↓

(c) On adding Sodium hydroxide to Copper salt pale blue coloured precipitate is obtained which is insoluble in excess of Sodium hydroxide. Ferrous salt solution gives a dirty green coloured precipitate with Sodium hydroxide which is insoluble in excess of NaOH.

$\underset{\text{blue}}{{\text{CuSO}_4}} + \underset{\text{colourless}}{2\text{NaOH}} \longrightarrow \underset{\text{pale blue ppt}}{\text{Cu(OH)}_2↓} + \underset{\text{colourless}}{\text{ Na}_2\text{SO}_4}$

$\underset{\text{pale green}}{{\text{FeSO}_4}} + \underset{\text{colourless}}{2\text{NaOH}} \longrightarrow \underset{\text{dirty green ppt}}{\text{Fe(OH)}_2↓} + \underset{\text{colourless}}{\text{Na}_2\text{SO}_4}$

(d) Sodium hydroxide on reaction with Fe(II) salt gives dirty green coloured precipitate, while with Fe(III) salt solution it forms reddish brown precipitate. Both precipitates are insoluble in excess NaOH.

Fe(II) salt:

$\underset{\text{pale green solution}}{{\text{FeSO}_4}} + 2\text{NaOH} \longrightarrow \underset{\text{dirty green ppt}}{\text{Fe(OH)}_2↓} + \underset{\text{colourless in solution}}{\text{Na}_2\text{SO}_4}$

Fe(III) salt :

$\underset{\text{yellow}}{{\text{FeCl}_3}} + \underset{\text{colourless}}{3\text{NaOH}} \longrightarrow \underset{\text{reddish brown ppt}}{\text{Fe(OH)}_3↓} + \underset{\text{colourless}}{3\text{NaCl}}$

(e) Ammonium hydroxide on reaction with lead nitrate gives a chalky white insoluble precipitate, and with ferrous nitrate forms a dirty green ppt.

$\underset{\text{colourless}}{{\text{Pb(NO}_3)_2}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white ppt}}{\text{Pb(OH)}_2↓} + 2\text{NH}_4\text{NO}_3$

$\underset{\text{colourless}}{{\text{Fe(NO}_3)_2}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{dirty green ppt}}{\text{Fe(OH)}_2↓} + \underset{\text{ colourless}}{2\text{NH}_4\text{NO}_3}$

How will you distinguish calcium nitrate and zinc nitrate solution?

Answer

When ammonium hydroxide (NH₄OH) is added to zinc nitrate solution [Zn(NO₃)₂], a gelatinous white ppt of zinc hydroxide [Zn(OH)₂] is obtained which is soluble in excess of NH₄OH.

$\underset{\text{colourless solution}}{{\text{ZnSO}_4}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white gelatinous ppt}}{\text{Zn(OH)}_2↓} + \underset{\text{colourless in solution}}{(\text{NH}_4)_2\text{SO}_4}$

(With excess NH₄OH ppt. dissolves)

$\text{Zn(OH)}_2 + (\text{NH}_4)_2\text{SO}_4 + 2\text{NH}_4\text{OH} \longrightarrow \underset{\underset{\text{zinc (II) sulphate}}{\text{Tetraammine}}}{[(\text{Zn(NH}_3)_4]\text{SO}_4} + 4\text{H}_2\text{O}$

On the other hand, calcium nitrate solution [Ca(NO₃)₂] does not give any ppt. even when excess of ammonium hydroxide is added.

You are provided with two reagent bottles marked A and B. One contains NH₄OH solution and the other contains NaOH solution. How will you identify them by a chemical test?

Answer

Reagent bottles A and B can identified by using Ca(NO₃)₂.
When NH₄OH solution is added to Ca(NO₃)₂ a white ppt is obtained.

$\underset{\text{colourless}}{{\text{Ca(NO}_3)_2}} + \underset{\text{caustic soda - colourless}}{2\text{NaOH}} \longrightarrow \underset{\text{white ppt}}{\text{Ca(OH)}_2} ↓ + \underset{\text{ colourless}}{2\text{NaNO}_3}$

On the other hand, addition of NH₄OH solution to Ca(NO₃)₂ gives no precipitate even when NH₄OH solution is added in excess. Thus, Ca(NO₃)₂ can be used to distinguish between NH₄OH and NaOH solution.

Write balanced equations for the following conversions:

(a) $\text{Zn(SO)}_4 \xrightarrow{\text{A}} \text{Zn(OH)}_2 \xrightarrow{\text{B}} \text{Na}_2\text{ZnO}_2$

(b) $\text{Cu(SO)}_4 \xrightarrow{\text{A}} \text{Cu(OH)}_2 \xrightarrow{\text{B}} \text{[Cu(NH}_3)_4]\text{SO}_4$

Answer

(a) When sodium hydroxide solution is added dropwise to zinc sulphate, a white gelatinous ppt of zinc hydroxide is obtained.

$\underset{\text{colourless}}{{\text{ZnSO}_4}} + \underset{\text{colourless}}{2\text{NaOH}} \longrightarrow \underset{\text{white gelatinous ppt}}{\text{Zn(OH)}_2} ↓ + \underset{\text{colourless}}{\text{Na}_2\text{SO}_4}$

On adding excess of NaOH solution, the ppt dissolves and a colourless solution is obtained.

$\text{Zn(OH)}_2 + \underset{\text{excess}}{2\text{NaOH}} \longrightarrow \underset{\underset{\text{colourless}}{\text{sodium zincate}}}{\text{Na}_2\text{ZnO}_2} + 2\text{H}_2\text{O}$

(b) When ammonia solution is added dropwise to copper sulphate, a pale blue ppt of copper hydroxide is obtained.

$\underset{\text{blue}}{{\text{CuSO}_4}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{pale blue ppt}}{\text{Cu(OH)}_2} ↓ + \underset{\text{colourless in solution}}{(\text{NH}_4)_2\text{SO}_4}$

On adding excess of ammonia solution, the ppt dissolves and a deep blue solution is obtained.

$\text{Cu(OH)}_2 + (\text{NH}_4)_2\text{SO}_4 + 2\text{NH}_4\text{OH} \longrightarrow \underset{\underset{\text{copper (II) sulphate}}{\text{tetraammine}}}{[(\text{Cu(NH}_3)_4]\text{SO}_4} + 4\text{H}_2\text{O}$

(a) What do you understand by amphoteric oxide?

(b) Give the balanced equations for the reaction with two different amphoteric oxides with a caustic alkali.

(c) Name the products formed.

Answer

(a) Amphoteric oxides and hydroxides are those compounds which react with both acids and alkalis to form salt and water.

(b) Balanced equations for the reaction of Zinc Oxide and Lead Oxide with Caustic Soda are given below:

ZnO + 2NaOH ⟶ Na₂ZnO₂ + H₂O

PbO + 2NaOH ⟶ Na₂PbO₂ + H₂O

(c) Sodium zincate [Na₂ZnO₂] and sodium plumbite [Na₂PbO₂] are the products formed.

On adding dilute ammonia solution to a colourless solution of a salt, a white gelatinous precipitate appears. This precipitate however dissolves on addition of excess of ammonia solution. Identify (choose from Na, Al, Zn, Pb, Fe)

(a) From the following list, identify which metal salt solution was used above?
    a, Al, Zn, Pb, Fe

(b) What is the formula of the white gelatinous precipitate obtained?

(c) Write the balanced reaction involved.

Answer

(a) Zinc (Zn) metal salt solution was used

(b) Zn(OH)₂

(c) When ammonia solution is added dropwise to zinc sulphate, a white gelatinous ppt of zinc hydroxide is obtained.

$\underset{\text{colourless solution}}{{\text{ZnSO}_4}} + 2\text{NH}_4\text{OH} \longrightarrow \underset{\text{white gelatinous ppt}}{\text{Zn(OH)}_2↓} + \underset{\text{colourless in solution}}{(\text{NH}_4)_2\text{SO}_4}$

On adding excess of ammonia solution, the ppt dissolves and a colourless solution is obtained.

$\text{Zn(OH)}_2 + (\text{NH}_4)_2\text{SO}_4 + 2\text{NH}_4\text{OH} \longrightarrow \underset{\underset{\text{zinc (II) sulphate}}{\text{Tetraammine}}}{[(\text{Zn(NH}_3)_4]\text{SO}_4} + 4\text{H}_2\text{O}$