Question

Fe (SCN)_n^3–n absorb colour because of the complex Fe(SCN) _n^3–n only, whereas Fe³⁺ and SCN^– both are colourless. In the given table the equilibrium concentration of Fe³⁺ and SCN^– and corresponding absorbance are given –

Exp. No. [Fe³⁺] [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 -

• A. 3
• B. 6
• C. 4
• D. 2

Answer 1

Answer: (A)

3

Answer 2

Fe³⁺ + n SCN^– Fe(SCN)_n^3–n

K_form =

Q [complex] = K_form [Fe³⁺] [SCN^–]ⁿ

\ log [complex] = log K_form + log [Fe³⁺]

+ n log [SCN^–]

[Fe³⁺] is constant, hence

log $\frac { [ \text { Complex } ] _ { 1 } } { [ \text { Complex } ] _ { 2 } }$= n log $\frac { \left[ \mathrm { SCN } ^ { - } \right] _ { 1 } } { \left[ \mathrm { SCN } ^ { - } \right] _ { 2 } }$

\ log 3/0.3 = n log $\frac { 10 } { ( 10 ) ^ { 2 / 3 } }$

1 = n × $\frac { 1 } { 3 }$

or n = 3