Why CoCl3.6NH3, .5NH3 and .4NH3 Give Different Amounts of AgCl
Chemistry · Coordination Compounds · NEET
Only the chloride ions written OUTSIDE the square bracket are free in water, so only they react with AgNO3 to make AgCl. Chloride bonded to the metal (inside the bracket) stays locked and gives no precipitate. That is why CoCl3.6NH3 = [Co(NH3)6]Cl3 gives 3 AgCl, CoCl3.5NH3 = [Co(NH3)5Cl]Cl2 gives 2, and CoCl3.4NH3 = [Co(NH3)4Cl2]Cl gives 1. Memory hook: "Inside the bracket is jailed, outside is free" — only free Cl makes AgCl, and the count goes 3, 2, 1.
Each cobalt-ammine complex written in true bracket form. Red Cl outside the bracket is free and gives white AgCl with AgNO3; Cl inside the bracket is bonded to Co and gives none. Count of free Cl goes 3, 2, 1.
Your doubts, answered
Why does CoCl3.6NH3 give 3 AgCl but CoCl3.4NH3 gives only 1?
Rewrite each in true form. CoCl3.6NH3 is [Co(NH3)6]Cl3: all 6 places on Co are taken by NH3, so all 3 Cl are outside the bracket and free. 3 free Cl react with Ag+ to give 3 AgCl. In CoCl3.4NH3 = [Co(NH3)4Cl2]Cl, two Cl are bonded to Co inside the bracket (locked) and only 1 Cl is outside. So only 1 AgCl forms. The amount of AgCl equals the number of Cl written outside the square bracket.
Which chloride precipitates with AgNO3 and which one does not?
Only the chloride ion OUTSIDE the square bracket precipitates. This outside Cl is held by primary valence (it is ionisable), so it breaks off in water and reacts with Ag+ to form white AgCl. The chloride INSIDE the bracket is held by secondary valence (bonded straight to the metal). It is non-ionisable, does not leave in water, and gives NO AgCl. Rule: count only the Cl outside the bracket.
How do I quickly find how many moles of AgCl a complex gives?
Step 1: Write the complex in [ ] form so you can see which Cl is bonded to the metal. Step 2: Count only the Cl ions sitting OUTSIDE the bracket. That number is the moles of AgCl per mole of complex (with excess AgNO3). Example: [Co(NH3)5Cl]Cl2 has 1 Cl inside (bonded to Co) and 2 Cl outside, so it gives 2 AgCl. Never count the Cl inside the bracket.
What is primary valence and secondary valence in this experiment?
Primary valence is the normal charge that is balanced by ions written outside the bracket (here the free Cl-). It is ionisable, so it splits off in water. Secondary valence is the number of groups bonded directly to the metal (here 6, all inside the bracket). It is non-ionisable and fixed. The AgCl test proves Werner's idea: only the primary-valence (outside) chloride reacts, showing the two kinds of bonding are different.
Why do CoCl3.6NH3, .5NH3 and .4NH3 have different colours too?
NCERT records the colours: CoCl3.6NH3 is yellow, .5NH3 is purple, and .4NH3 is green (or violet for its isomer). The colour changes because the set of ligands around Co changes (more NH3 replaced by Cl inside the bracket), which changes the crystal field splitting and the light absorbed. For this concept just remember the AgCl counts 3, 2, 1; the colours are a bonus NEET sometimes checks.
⚠️ The NEET trap ✗ Counting the total number of Cl atoms in the formula, so a student says CoCl3.5NH3 gives 3 AgCl because it has three chlorines. ✓ Only the Cl OUTSIDE the bracket counts. CoCl3.5NH3 = [Co(NH3)5Cl]Cl2 has 1 Cl bonded to Co (inside, no precipitate) and 2 Cl outside, so it gives 2 AgCl, not 3. 🧠 See a Cl in a coordination compound? Ask 'inside or outside the bracket?' Inside = jailed (no AgCl). Outside = free (makes AgCl).
Real NEET questions
NEET 2017
The correct order of the stoichiometries of AgCl formed when AgNO3 in excess is treated with the complexes CoCl3.6NH3, CoCl3.5NH3, CoCl3.4NH3 respectively is:
A · 1 AgCl, 3 AgCl, 2 AgCl
B · 3 AgCl, 1 AgCl, 2 AgCl
C · 3 AgCl, 2 AgCl, 1 AgCl ✓
D · 2 AgCl, 3 AgCl, 1 AgCl
Solution: Write each addition compound in true bracket form and count only the Cl outside the bracket (ionisable, held by primary valence). CoCl3.6NH3 = [Co(NH3)6]Cl3 has 3 free Cl outside so gives 3 AgCl. CoCl3.5NH3 = [Co(NH3)5Cl]Cl2 has 1 Cl bonded to Co inside and 2 free outside so gives 2 AgCl. CoCl3.4NH3 = [Co(NH3)4Cl2]Cl has 2 Cl inside and 1 free outside so gives 1 AgCl. Order = 3, 2, 1, option C.
NEET 2025
Out of the following complex compounds, which compound will have the minimum conductance in solution?
A · [Co(NH3)6]Cl3
B · [Co(NH3)5Cl]Cl2
C · [Co(NH3)3Cl3] ✓
D · [Co(NH3)4Cl2]Cl
Solution: Conductance depends on the number of free ions, which is the same idea as the AgCl test: only Cl outside the bracket ionises. [Co(NH3)6]Cl3 gives 4 ions (1 cation + 3 Cl). [Co(NH3)5Cl]Cl2 gives 3 ions. [Co(NH3)4Cl2]Cl gives 2 ions. [Co(NH3)3Cl3] has all three Cl bonded to Co inside the bracket, so it is a neutral molecule with 0 free ions and the lowest conductance. It would also give 0 AgCl. Answer C.
ReNEET 2026
A 1:3 electrolyte in aqueous solution is:
A · [CoCl2(NH3)4]Cl
B · [CoCl(NH3)5]Cl2
C · [Co(NH3)6]Cl3 ✓
D · [Co(NH3)3(NO2)3]
Solution: A 1:3 electrolyte breaks into 1 cation and 3 anions. Count the Cl outside the bracket (the ionisable ones). [Co(NH3)6]Cl3 gives [Co(NH3)6]3+ and 3 Cl-, that is 1 cation : 3 anions = 1:3. It would also give 3 AgCl with AgNO3. The others give 1:1, 1:2, or are neutral non-electrolytes. Answer C.
Solved Coordination Compounds NEET PYQs
Try the real previous-year questions from this chapter — each with the answer and a full solution.
How many moles of AgCl does 1 mole of CoCl3.6NH3 give?
3 moles of AgCl. CoCl3.6NH3 = [Co(NH3)6]Cl3, so all three chloride ions are outside the bracket and free to react with Ag+.
Does the chloride bonded to cobalt give AgCl?
No. Chloride bonded to the metal sits inside the square bracket, is non-ionisable, and does not leave in water, so it gives no AgCl.
What test is used to count the free chloride ions?
Add excess AgNO3 to the complex solution. Each free (outside-the-bracket) Cl- makes one white AgCl precipitate, so the moles of AgCl equal the moles of free Cl-.
Why does this matter for NEET?
NEET repeatedly asks the AgCl count, molar conductance, and electrolyte ratio (1:1, 1:2, 1:3) of these cobalt-ammine complexes. All three use the same one rule: count only the Cl outside the bracket.
Why do CoCl3.4NH3 appear as two compounds with the same formula?
[Co(NH3)4Cl2]Cl can have the two inside Cl as cis (green) or trans (violet). Same formula, different arrangement, so they are isomers. Both still give 1 AgCl because each has only 1 Cl outside.
