Analytical Chemistry — Use of Ammonium & Sodium Hydroxide

Question 1

Sodium hydroxide solution is added first in a small amount, then in excess to the aqueous salt solutions of:

1. copper [II] sulphate
2. zinc nitrate
3. lead nitrate
4. iron [III] sulphate

State in each case :

(i) the colour of the precipitate when NaOH is added in a small quantity;

(ii) the nature of the precipitate [i.e. soluble or insoluble] when NaOH is added in excess.

Answer

(i) The colour of the precipitate when NaOH is added in a small quantity are as follows —

1. Pale blue — copper [II] sulphate
2. White — zinc nitrate
3. Chalky white — lead nitrate
4. Reddish brown — iron [III] sulphate

(ii) the nature of the precipitate [i.e. soluble or insoluble] when NaOH is added in excess are as follows —

1. Insoluble — copper [II] sulphate
2. Soluble — zinc nitrate
3. Soluble — lead nitrate
4. Insoluble — iron [III] sulphate

Question 2

Write balanced equations for — (a) Aluminium (b) Zinc — is warmed with NaOH [caustic soda] soln.

Answer

1. Aluminium
   2Al + 2NaOH + 2H₂O ⟶ 2NaAlO₂ + 3H₂

2. Zinc
   Zn + 2NaOH ⟶ Na₂ZnO₂ + H₂

Question 1

The questions below refers to the following salt solutions listed A to F:

A: Copper nitrate

B: Iron [II] sulphate

C: Iron [III] chloride

D: Lead nitrate

E: Magnesium sulphate

F: Zinc chloride

(i) Which soln. becomes a deep/inky blue colour when excess of ammonium hydroxide is added to it.

(ii) Which solution gives a white precipitate with excess ammonium hydroxide solution.

Answer

(i) A: Copper nitrate becomes a deep/inky blue colour when excess of ammonium hydroxide is added to it.

(ii) D: Lead nitrate gives a white precipitate with excess ammonium hydroxide solution.

Question 1

From the list of substances given — Ammonium sulphate, Lead carbonate, Chlorine, Copper nitrate, Ferrous sulphate — State a solution of the compound which gives a dirty green precipitate with sodium hydroxide.

Answer

Ferrous sulphate gives a dirty green precipitate with sodium hydroxide.

Question 2

Write a balanced equation for the reaction between aluminium oxide and sodium hydroxide solution.

Answer

Al₂O₃ + 2NaOH ⟶ 2NaAlO₂ + H₂O

Question 3

Give one test to distinguish between the following:

Iron [III] chloride soln. and Copper chloride soln.

Answer

Ammonium hydroxide can be used to distinguish between the two. Iron [III] chloride soln. forms reddish brown ppt. of Fe(OH)₃ which is insoluble in excess of ammonium hydroxide whereas copper chloride soln. forms a blue ppt. of Cu(OH)₂ which is soluble in excess of ammonium hydroxide.

Question 1

The salt which in soln. gives a pale green precipitate with NaOH solution and a white ppt. with BaCl₂ soln. is :

1. Iron [III] sulphate
2. Iron [II] sulphate
3. Iron [II] chloride
4. Iron [III] chloride

Answer

Iron [II] sulphate gives a pale green precipitate with NaOH solution and a white ppt. with BaCl₂ soln.

Question 1

Find the odd one with reasons [valency is not a criterion] :

1. Al(OH)₃
2. Pb(OH)₂
3. Mg(OH)₂
4. Zn(OH)₂

Answer

Mg(OH)₂
Reason — All others are amphoteric hydroxides.

Question 2

Identity the substance P based on the information given :

The deliquescent salt P, turns yellow on dissolving in water, and gives a reddish brown precipitate with sodium hydroxide solution.

Answer

Ferric chloride [Fe(Cl)₃]

Question 1

Give an equation for —

1. ZnO reacts with NaOH solution.
2. Conversion of – Zn(NO₃)₂ to Zn(OH)₂

Answer

1. ZnO + 2NaOH ⟶ Na₂ZnO₂ + H₂O
2. Zn(NO₃)₂ + 2NaOH ⟶ Zn(OH)₂ + 2NaNO₃

Question 2

Select the correct answer from A, B, C —

A: Sodium hydroxide soln.
B: A weak acid
C: Dilute sulphuric acid.

The solution which with zinc sulphate solution will give a white precipitate.

Answer

A: Sodium hydroxide soln

Question 1

Choose the correct answer :

Hydroxide of this metal is soluble in NaOH solution.

1. Magnesium
2. Lead
3. Silver
4. Copper

Answer

Lead

Question 2

Sodium hydroxide solution is added to the solutions containing the ions mentioned in List 1.

List 1 —

1. Pb²⁺
2. Fe²⁺
3. Zn²⁺
4. Fe³⁺
5. Cu²⁺
6. Ca²⁺

The precipitates obtained are in list 2 —

A: Reddish brown
B: White insoluble in excess
C: Dirty green
D: White soluble in excess
E: White soluble in excess
F: Blue.

Match list 1 with list 2

Answer

Question 3

Give balanced equations for —

(i) Zinc oxide dissolves in NaOH

(ii) Zinc is heated with NaOH solution

Answer

(i) ZnO + 2NaOH ⟶ Na₂ZnO₂ + H₂O

(ii) Zn + 2NaOH ⟶ Na₂ZnO₂ + H₂

Question 1

Name: The gas evolved on reaction of aluminium with boiling concentrated caustic alkali solution.

Answer

Hydrogen gas
2Al + 2NaOH + 2H₂O ⟶ 2NaAlO₂ + 3H₂

Question 2

State one observation for:

1. Excess NH₄OH soln. is added to Pb(NO₃)₂
2. NaOH soln. is added to FeCl₃ soln. in excess.

Answer

1. Chalky white insoluble precipitate is obtained.
   Pb(NO₃)₂ + 2NH₄OH ⟶ 2NH₄NO₃ + Pb(OH)₂ ↓

2. Reddish brown insoluble precipitate is obtained.
   FeCl₃ + 3NaOH ⟶ 3NaCl + Fe(OH)₃ ↓

Question 1

State two relevant observations for the reaction:

Ammonium hydroxide solution is added to copper (II) nitrate solution in small quantities and then in excess.

Answer

Two observations are:

1. A bluish white ppt. is formed when small amount of ammonium hydroxide is added.
2. This precipitate turns deep blue when excess of ammonium hydroxide solution is added.

Question 1

State your observation: When excess sodium hydroxide is added to calcium nitrate solution.

Answer

White precipitate is obtained which is almost insoluble in excess of NaOH.

Question 1

To a salt solution 'Y' a small quantity of NH₄OH solution is added slowly and then in excess. A pale blue precipitate is formed which dissolves in ex­cess to form a clear inky blue solution. Identify the posi­tive ion in the salt 'Y'

Answer

Cu²⁺ ion.

Question 1

State your observations when ammonium hydroxide solution is added drop by drop and then in excess to each of the following:

(i) copper sulphate solution

(ii) zinc sulphate solution.

Answer

(i) Pale blue precipitate of Cu(OH)₂ is formed, which becomes deep blue when excess of ammonium hydroxide is added.

CuSO₄ + 2NH₄OH ⟶ (NH₄)₂SO₄ + Cu(OH)₂ ↓

Cu(OH)₂ + (NH₄)₂SO₄ + 2NH₄OH [in excess] ⟶ 4H₂O + [Cu(NH₃)₄]SO₄

(ii) Gelatinous white precipitate of zinc hydroxide is formed which becomes colourless solution on adding excess of ammonium hydroxide.

ZnSO₄ + 2NH₄OH ⟶ (NH₄)₂SO₄ + Zn(OH)₂ ↓

Zn(OH)₂ +(NH₄)₂SO₄ + 2NH₄OH [in excess] ⟶ 4H₂O + [Zn(NH₃)₄]SO₄

Question 1

State one relevant observation – Action of sodium hydroxide solution on iron [II] sulphate solution.

Answer

A dirty green precipitate is formed which is insoluble in excess of sodium hydroxide solution.

Question 2

How will you distinguish between — Ammonium Hydroxide & Sodium Hydroxide using CuSO₄

Answer

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.

Question 1

Choose the correct answer from the options given :

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 — concentration of OH^1- ions is low and so ppt. cannot be formed.

Question 2

Write a balanced chemical equation for :

(i) Reaction of sodium hydroxide solution with iron [III] chloride solution.

(ii) Reaction of zinc with potassium hydroxide solution.

Answer

(i) FeCl₃ + 3NaOH ⟶ 3NaCl + Fe(OH)₃

(ii) Zn + 2KOH ⟶ K₂ZnO₂ + H₂

Question 3

State one relevant observation : Lead nitrate solution is treated with sodium hydroxide soln. drop wise, till it is in excess.

Answer

Chalky white precipitate is formed which is soluble in excess of sodium hydroxide and forms a colourless solution.

Question 1

Write balanced chemical equation for —

Ammonium hydroxide is added to ferrous sulphate solution.

Answer

2NH₄OH + FeSO₄ ⟶ (NH₄)₂SO₄ + Fe(OH)₂

Question 2

Copper [II] sulphate solution reacts with NaOH solution to form a precipitate of Cu(OH)₂. State it's colour.

Answer

Pale blue precipitate is formed.

Question 1

State one relevant observation for the reaction : Addition of excess ammonium hydroxide into copper sulphate solution.

Answer

Deep blue or inky blue solution is formed when excess of ammonium hydroxide is added to copper sulphate solution.

Question 1

Salts of ............... [normal/transitional] elements are generally coloured. From the ions K¹⁺, Cr³⁺, Fe²⁺, Ca²⁺, SO₃^2-, MnO₄^1-, NO₃^1- the ions generally coloured are ...............

Answer

Salts of transitional elements are generally coloured. From the ions K¹⁺, Cr³⁺, Fe²⁺, Ca²⁺, SO₃^2-, MnO₄^1-, NO₃^1- the ions generally coloured are Cr³⁺, Fe²⁺, MnO₄^1- .

Question 2

The hydroxide which is soluble in excess of NaOH is ............... [Zn(OH)₂/Fe(OH)₃/Fe(OH)₂]

Answer

The hydroxide which is soluble in excess of NaOH is Zn(OH)₂

Question 3

The salt which will not react with NH₄OH solution is ............... [ZnCl₂/CuCl₂/NH₄Cl/FeCl₂]

Answer

The salt which will not react with NH₄OH solution is NH₄Cl

Question 4

The substance/s which react with hot conc. NaOH soln. and undergoes a neutralization reaction ............... [Al₂O₃/Al/Al(OH)₃]

Answer

The substance/s which react with hot conc. NaOH soln. and undergoes a neutralization reaction Al₂O₃.

Question 5

To distinguish soluble salts of zinc and lead, ............... [NaOH/NH₄OH] can be used.

Answer

To distinguish soluble salts of zinc and lead NaOH can be used.

Question 6

Oxides and hydroxides of certain metals i.e ............... [iron/ zinc/copper/aluminium/magnesium/lead] are amphoteric and react with ............... [acids/alkalis/acids and alkalis] to give salt and water.

Answer

Oxides and hydroxides of certain metals i.e zinc, aluminium, lead are amphoteric and react with acids and alkalis to give salt and water.

Question 1

Answer

Question 2

Answer

Question 3

Answer

Question 4

Answer