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Standard XII

Chemistry

Solubility Product

To avoid prec...

Question

To avoid precipitation of hydroxides of $N i^{2 +}, C o^{2 +}$ and $M n^{2 +}$ along with those of the third group cations, the solutions should be:

heated with few drops of concentrated

H N O_{3}

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boiled with excess of

N H_{4} C l

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concentrated to small volume

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none of the above

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Solution

The correct option is B boiled with excess of $N H_{4} C l$
The function of strong electrolyte $N H_{4} C l$ is to suppress the ionisation of $N H_{4} O H$ , so that the concentration of $O H^{-}$ ions in the solution is decreased. But, it is sufficient to precipitate the third group basic radicals because the solubility product of group III hydroxides is lower than IV, V and VI group hydroxides. $N H_{4} O H$ should slightly be in excess of the precipitant. Upon boiling the solution, $C r (O H)_{3}$ is precipitated.

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Similar questions

Q. To avoid the precipitation of hydroxides of

N i^{2 +}

C o^{2 +}

Z n^{2 +}

, and

M n^{2 +}

along with those of

F e^{3 +}

A l^{3 +}

, and

C r^{3 +}

the third group solution should be :

Q. To precipitate group III cations

N H_{4} C l

should be added to the solution before the addition of ammonium hydroxide. Explain why.

Q. Which of these is the correct group reagent for

M n^{2 +}, C o^{2 +}, Z n^{2 +} & N i^{2 +}

group cations?

In qualitative analysis,

{I I I}^{r d}

group includes

F e^{3 +}, A l^{3 +}

and

C r^{3 +}

. The group reagent is ammonium hydroxide in the presence of ammonium chloride.

N H_{4} C I

is added to suppress the ionisation of

N H_{4} O H

so that only the

I I I^{r d}

group radicals are precipitated as their insoluble hydroxide and not the

I V^{t h}

and

V^{t h}

group radicals as the solubility products of their hydroxides are much higher. Before adding group reagent to the filtrate of IInd group, one drop of concentrated

H N O_{3}

(oxidising agent) is added and the content is boiled for 2-3 minutes and then cool because the solubility product of

F e (O H)_{2}

is very high as compared to

F e (O H)_{3}

. When a light bluish-green crystalline double salt-containing cations of III

^{r d}

group and zero groups are analysed. it responds to the following tests/reactions.
(a) Its aqueous solution gives a reddish-brown precipitate with an alkaline solution of potassium tetraiodomercurate (II)
(b) Its aqueous solution after boiling with conc.

H N O_{3}

gives a reddish-brown precipitate with sodium hydroxide.
(c) The solution of reddish-brown precipitate in oil.HCI gives blood red colouration with ammonium sulphocyanide and Prussian blue with potassium ferrocyanide.
(d) It's a very dilute solution gives a white precipitate with

(C H_{3} C O O)_{2} P b

solution and a white precipitate are only soluble in hot ammonium acetate.

Why one mL conc.

H N O_{3}

is added to

I I^{n d}

group filtrate before proceeding for

I I I^{r d}

group radicals?

Q. A few drops of concentrated ammonia solution added to a calcium bicarbonate solution causes a white precipitate. The precipitate formed is:

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To avoid precipitation of hydroxides of Ni2+, Co2+ and Mn2+ along with those of the third group cations, the solutions should be:

To avoid precipitation of hydroxides of $N i^{2 +}, C o^{2 +}$ and $M n^{2 +}$ along with those of the third group cations, the solutions should be: