Name: Acid-base Indicators
Uploaded: 2019-07-19
Duration: 9 min 55 s
Description: This video explains indicators as a chemical species used to distinguish between acids and bases. Commonly used natural indicators and also a universal indic...

Question

In qualitative analysis, explain why the cations of group

II

are precipitated in acidic medium while those of group

IV

in alkaline medium.

Accepted Answer

Group II: sparingly soluble sulphides derived from ${Hg}^{2 +}, {Pb}^{2 +}, {Cu}^{2 +}, {Cd}^{2 +}$ and ${Bi}^{3 +}$ ions (and in addition the sparingly soluble covalent sulphides ${As}_{2} S_{3}, {Sb}_{2} S_{3}$ and $SnS$ ) are precipitates by the passage by the passage of hydrogen sulphide $(H_{2} S)$ through an acidic solution whose concentration of hydrogen ion is small.

Sulphidic $Ksp$ at $25^{ο} C$

$HgS$ $3 \times 10^{- 52}$

$PbS$ $8 \times 10^{- 28}$

$CuS$ $8 \times 10^{- 36}$

$CdS$ $1 \times 10^{- 27}$

${Bi}_{2} S_{3}$ $1 \times 10^{- 96}$

The $Ksp$ value of group $II$ sulphide is quite low in comparison to $Ksp$ values of group $IV$ sulphides, hence they do not get precipitated in acidic medium, where $S^{2 -}$ ion concentration is quite low.

In qualitative analysis, explain why the cations of group $II$ are precipitated in acidic medium while those of group $IV$ in alkaline medium.

Chapter-Wise 3D Chemistry Videos from Dr. H.C. Srivastava ISC Nootan Chemistry Volume 1 for Class XI

Acid-base Indicators

Introduction to Equilibrium

Condition for Precipitation

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