Question

Select the correct statement(s)
A. In group III, `Fe^(3+)` and `Cr^(3+)` can be differentiated by increasing `NH_(4)^(+)` ion concentration
B. In `V^(th),Na_(2)CO_(3)` is added to precipitate out only the carbonates of `Ba^(2+),Sr^(2+)` and `Ca^(2+)`
C. Like brown ring test, diphenylamine test is given only by salts containing `NO_(3)^(-)`
D. Sodium chloride on heating with aqueous solution of `K_(2)Cr_(2)O_(7)` and concentrated `H_(2)SO_(4)` produces deep vapour

Answer 1

Correct Answer - A
(A)`Cr^(3+)+3NH_(3)+3H_(2)OtoCr(OH)_(3)darr+3NH_(4)^(+)`
The reaction is reversible, on addition of `NH_(4)^(+)` shifts to backward direction.Thus if excess of `NH_(4)^(+)` salt is added, then precipitation of `Cr(OH)_(3)` will not take place. However because of very small `k_(sp` of iron `(III)` hydroxide complete precipitation will take place even in the presence of ammonium salts. `(k_(sp)=3.8xx10^(-38))`
(B)Concentration of `CO_(3)^(2-)` provided by `Na_(2)CO_(3)` in aqueous solution is just sufficient to precipitate `Mg^(2+)` ion as `MgCO_(3)` along with `Ba^(2+),Ca^(2+)` and `Sr^(2+)` as their carbonates.
(C )The oxidising anions like `MnO_(4)^(-),Cr_(2)O_(7)^(2-), CIO_(4)^(-)` etc also respond to this test.
(D)`CrO_(2)Cl_(2) +2H_(2)O to H_(2)CrO_(4)(aq)+2HCl`