Analytical Chemistry – Use of NaOH and NH4OH

advertisement
Page |1
Analytical Chemistry – Use of NaOH and
NH4OH (Solved)
Question .1. Using Sodium hydroxide solution, how would you distinguish :
(i) Zinc nitrate (or sulphate)solution and Calcium nitrate solution?
(ii) Iron (II) chloride from Iron (III) chloride?
(iii) Lead hydroxide from Magnesium hydroxide?
Answer : (i)
Zinc nitrate (or sulphate)
Calcium nitrate
When Sodium hydroxide solution is added a
white precipitate is formed which is soluble in
excess of sodium hydroxide.
When Sodium hydroxide solution is added a
white precipitate is formed which is insoluble
in excess of Sodium hydroxide.
(ii)
Iron (II) chloride
Iron (III) chloride
When sodium hydroxide is added, dirty green
precipitate is formed which changes to
reddish brown after some time.
When sodium hydroxide solution is added, a
reddish brown precipitate is formed.
(iii)
Lead hydroxide
Magnesium hydroxide
When sodium hydroxide is added, lead
hydroxide is dissolved in it.
When sodium hydroxide is added, there is no
visible reaction i.e. it remains insoluble.
Question .2. (i) Sodium hydroxide solution is added to solution A. A white precipitate is formed
which is insoluble in excess of Sodium hydroxide solution. What is the metal ion present in
solution A?
(ii) When Ammonium hydroxide solution is added to solution B a pale blue precipitate is formed.
This pale blue precipitate dissolves in excess of Ammonium hydroxide giving an inky blue
solution. What is the cation present in solution B? What is the probable colour of solution B?
Answer : (i) Calcium ion [Ca2+]
Page |2
(ii) Copper ion [Cu2+]. The colour of solution ‘B’ is blue.
Question .3. You are given the three white powders Calcium carbonate, Lead carbonate and Zinc
carbonate. Describe the tests you would carry out in solution to identify the metal in each of the
above compounds. Indicate clearly how you would prepare the solutions for the tests.
Answer :
Calcium carbonate
Lead carbonate
Zinc carbonate
Calcium carbonate is dissolved in
dilute Nitric acid and then
Ammonium hydroxide is added to
it. No visible reaction is seen.
Lead carbonate is dissolved in
dilute Nitric acid and then
Ammonium hydroxide is added to
it. A white precipitate is formed
which is insoluble in excess.
Zinc carbonate is dissolved in
dilute Nitric acid and then
Ammonium hydroxide is added to
it. A white precipitate is formed
which is soluble in excess.
Question .4. Write an equation for the following reaction :
Copper sulphate solution is added to Sodium hydroxide solution.
Answer : CuSO4 + 2NaOH → Cu(OH)2 + Na2SO4.
Question .5. Three test-tubes contain Calcium nitrate solution, Zinc nitrate solution and Lead
nitrate solution respectively. Each solution is divided into two portions (i) and (ii). Describe the
effect of :
(i) Adding Sodium hydroxide to each portion in turn till it is in excess.
(ii) Adding Ammonium hydroxide to each portion in turn till it is in excess.
Answer : (i) Sodium Hydroxide
Small amount
In excess
Calcium nitrate
White precipitate
Insoluble
Zinc nitrate
White precipitate
Soluble
Lead nitrate
White precipitate
Soluble
(ii) Ammonium Hydroxide
Page |3
Small amount
In excess
Calcium nitrate
No visible reaction
No change
Zinc nitrate
White precipitate
Soluble
Lead nitrate
White precipitate
Insoluble
Question .6. State what do you observe when : Ammonium hydroxide is added to Iron (III)
sulphate solution.
Answer : A reddish brown precipitate is formed.
Question .7. The following table shows the tests a student performed two aqueous solutions A
and B. Write down the observations (i) and (ii) that were made.
Test
Observation
Conclusion
(i) To solution A, Sodium
hydroxide solution was
added.
(i)
A contains
Fe3+ ions
(ii) To solution B, Ammonium
hydroxide solution was added
slowly till in excess.
(ii)
B contains
Cu2+ ions
Answer : (i) Reddish brown coloured precipitate is appeared.
(ii) A pale blue or bluish white precipitate is formed which dissolves in excess of Ammonium hydroxide and give deep
blue / intense blue / inky blue solution.
Question .8. How would you distinguish between Zn2+ and Pb2+ using ammonium hydroxide
solution?
Answer : Solution:
Zn2+
Pb2+
Page |4
On adding NH4OH drop by drop to a solution of Zinc
salt a white precipitate is formed which is soluble in
excess of NH4OH.
On adding NH4OH drop by drop to a solution of lead
salt a white precipitate is formed which is insoluble in
excess of NH4OH.
Question .9. Write observations and balanced equations for the following reactions.
(i) Sodium hydroxide is added drop wise till in excess to a solution of Zinc sulphate.
(ii) Ammonium hydroxide is added first in a small quantity and then in excess to a solution of
copper sulphate.
(iii) Excess of Ammonium hydroxide is added to a substance obtained by adding hydrochloric
acid in silver nitrate solution.
Answer : (i) When sodium hydroxide solution is added drop by drop to zinc sulphate solution a white precipitate is
formed which is soluble in excess of sodium hydroxide.
ZnSO4 + 2NaOH → Zn(OH)2 + Na2SO4
Zn(OH)2 + 2NaOH → Na2ZnO2 + 2H2O.
(ii) When NH4OH solution is added drop by drop to copper sulphate solution a pale blue / bluish white precipitate is
formed which is soluble in excess of NH4OH and deep blue / inky blue solution is formed.
CuSO4 + 2NH4OH → Cu(OH)2 + (NH4)2SO4
Cu(OH)2 + 4NH4OH → Cu(NH3)4(OH)2 + 4H2O.
(iii) When hydrochloric acid is added to silver nitrate solution a white precipitate is obtained which is soluble in excess
of Ammonium hydroxide solution.
AgNO3 + HCl → AgCl + HNO3
AgCl + 2NH4OH → [Ag(NH3)2]Cl + 2H2O.
Question .10. Sodium hydroxide solution is added first in a small quantity, then in excess to the
aqueous salt solution of copper (II) sulphate, zinc nitrate, lead nitrate, calcium chloride and iron
(III) sulphate. Copy the following table and write the colour of the precipitate in (i) to (v) and the
nature of the precipitate (soluble or insoluble) in (vi) to (x).
Colour of precipitate when
NaOH is added in a small
Nature of precipitate
(soluble or insoluble) when
Page |5
Aqueous salt solution.
quantity.
NaOH is added in excess.
Copper (II) sulphate
(i)
(vi)
Zinc nitrate
(ii)
(vii)
Lead nitrate
(iii)
(viii)
Calcium chloride
(iv)
(ix)
Iron (II) sulphate
(v)
(x)
Answer :
Aqueous salt solution.
Colour of precipitate when
NaOH is added in a small
quantity.
Nature of precipitate (soluble
or insoluble) when NaOH is
added in excess.
Copper (II) sulphate
(i) Light Blue
(vi) Insoluble
Zinc nitrate
(ii) White
(vii) Soluble
Lead nitrate
(iii) White
(viii) Soluble
Calcium chloride
(iv) White
(ix) Insoluble
Iron (II) sulphate
(v) Dirty green
(x) Insoluble
Question .11. The questions (i) to (v) refer 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 two solutions will give a white precipitate when treated with dilute hydrochloric acid
followed by barium chloride solution?
(ii) Which two solutions will give a white precipitate when treated with dilute nitric acid followed
by silver nitrate solution?
Page |6
(iii) Which solution will give a white precipitate when either dilute hydrochloric acid or dilute
sulphuric acid is added to it?
(iv) Which solution becomes a deep/inky blue colour when excess of ammonium hydroxide is
added to it?
(v) Which solution gives a white precipitate with excess ammonium hydroxide solution?
Answer : (i) B {iron (II) sulphate} and E [magnesium sulphate]
(ii) C [iron (III) chloride] and F [zinc chloride]
(iii) D [lead nitrate]
(iv) A [copper nitrate]
(v) D [lead nitrate].
Question .12. 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
Question .13. Write a balanced equation for the reaction between – aluminium oxideand sodium
hydroxide solution.
Answer : Al2O3 + 2NaOH → 2NaAlO2 + H2O.
Question .14. Give one test to distinguish between the following : - Iron (III) chloride solution and
copper chloride solution.
Answer :
Iron (III) chloride solution
When NaOH solution is added a
dirty green precipitate of iron (III)
hydroxide is formed.
Copper chloride solution
When NaOH solution is added a
bluish precipitate of copper (II)
hydroxide is formed.
Page |7
Related documents
Download