Fe (SCN) n 3–n absorb colour because of the complex Fe(SCN) n 3–n only, whereas Fe 3+ and SCN – both are colourless. In the given table the equilibrium concentration of Fe 3+ and SCN – and corresponding absorbance are given –
Exp. No. [Fe 3+ ] [SCN – ] Absorbance
1 3×10 –3 (M) (10 M) 3
2 3×10 –3 (M) (10) 2/3 (M) 0.3
Considering absorbance proportional to concentration of the complex, the value of n is -
Text Solution
Verified by ExpertsA
Fe 3+ + n SCN – 
Fe(SCN) n 3–n
K form = 
[complex] = K form [Fe 3+ ] [SCN – ] n ∴ log [complex] = log K form + log [Fe
3+ ]
+ n log [SCN – ]
[Fe 3+ ] is constant, hence
log 
= n log 
∴ log 3/0.3 = n log 
1 = n × 
or n = 3
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