Determine the formula and charge of the complex ion formed from Fe3+ and SCN- in equimolar solutions, given maximum color intensity at a 1:1 mole ratio.

"Equimolar solutions" means that the molar concentrations of $Fe^{3+}(aq)$ and $SCN^{-}(aq)$ are equal. Let this concentration be $C \text{ mol/cm}^3$.

Volume of $Fe^{3+}$ ions = $5 \text{ cm}^3$
Moles of $Fe^{3+} = \text{Concentration} \times \text{Volume} = C \times 5 \text{ mol}$

Volume of $SCN^{-}$ ions = $5 \text{ cm}^3$
Moles of $SCN^{-} = \text{Concentration} \times \text{Volume} = C \times 5 \text{ mol}$

Maximum colour intensity indicates the stoichiometric ratio at which the complex is formed.
The ratio of moles of $Fe^{3+}$ to $SCN^{-}$ is:

This means that one $Fe^{3+}$ ion combines with one $SCN^{-}$ ion to form the complex.

The central metal ion is $Fe^{3+}$.
The ligand is $SCN^{-}$.
Since the ratio is 1:1, the complex will contain one $Fe$ and one $SCN$.
The charge of the complex is the sum of the charges of the metal ion and the ligand:

Therefore, the formula of the complex ion is $[Fe(SCN)]^{2+}$.