On the basis of following observations arrange the following compounds in increasing order of mol of AgCl precipitated per mol of the compound with excess of \( AgNO_3 \)
|
|
Formula |
Secondary valences |
Solution conductivity |
|
(A) |
\( PdCl_2.4NH_3 \) |
4 |
1:2 electrolyte |
|
(B) |
\( NiCl_4.2K \) |
4 |
2:1 electrolyte |
|
(C) |
\( CoCl_3.4NH_3 \) |
6 |
1:1 electrolyte |
|
(D) |
\( CrCl_3.6H_2O \) |
6 |
1:3 electrolyte |
Choose the correct answer from the options given below:
Answer & explanation
Correct answer: option 2
The correct answer is Option (2) → (B), (C), (A), (D)
In Werner’s theory, the secondary valence is the number of groups bound directly to the metal (the coordination number). Chloride ions inside the coordination sphere are held firmly and are not precipitated; only the chloride ions outside it (ionisable chloride) give AgCl with silver nitrate. The electrolyte type tells how many ions the compound gives in solution:
(A) \( \ce{PdCl2.4NH3} \): secondary valence 4, filled by the four \( \ce{NH3} \). Formula \( \ce{[Pd(NH3)4]Cl2} \), a 1:2 electrolyte:
\( \displaystyle \ce{[Pd(NH3)4]Cl2 + 2AgNO3 -> [Pd(NH3)4](NO3)2 + 2AgCl v} \): 2 mol AgCl.
(B) \( \ce{NiCl4.2K} \): secondary valence 4, filled by the four \( \ce{Cl-} \). Formula \( \ce{K2[NiCl4]} \), a 2:1 electrolyte (two \( \ce{K+} \), one \( \ce{[NiCl4]^2-} \)); no chloride outside the sphere: 0 mol AgCl.
(C) \( \ce{CoCl3.4NH3} \): secondary valence 6, so four \( \ce{NH3} \) and two \( \ce{Cl-} \) are bound. Formula \( \ce{[CoCl2(NH3)4]Cl} \), a 1:1 electrolyte: 1 mol AgCl.
(D) \( \ce{CrCl3.6H2O} \): secondary valence 6, filled by six \( \ce{H2O} \). Formula \( \ce{[Cr(H2O)6]Cl3} \), a 1:3 electrolyte: 3 mol AgCl.
In increasing order of AgCl precipitated:
\( \displaystyle \ce{K2[NiCl4]}\,(0) \lt \ce{[CoCl2(NH3)4]Cl}\,(1) \lt \ce{[Pd(NH3)4]Cl2}\,(2) \lt \ce{[Cr(H2O)6]Cl3}\,(3) \)
Shortcut: \( \ce{K2[NiCl4]} \) gives no AgCl and \( \ce{[Cr(H2O)6]Cl3} \) gives the most, so the order starts with (B) and ends with (D): only Option (2).
NCERT (2026-27 edition):
Werner’s Theory of Coordination Compounds – “… 2. The primary valences are normally ionisable and are satisfied by negative ions.
3. The secondary valences are non ionisable. These are satisfied by neutral molecules or negative ions. The secondary valence is equal to the coordination number and is fixed for a metal.”