The molar ratio of Fe⁺⁺ to Fe⁺⁺⁺ in a mixture of FeSO4 and Fe2(SO₄)₃ having equal number of sulphate ion in both ferrous and ferric sulphate is

1 mole of FeSO₄ and Fe₂(SO4)₃ has total one and three moles respectively of sulphate ions.

For same number of sulphate ions, 3 moles of FeSO4 are equivalent to 1 mole of F2(SO₄)₃

$\begin{array}{l}\mathrm{a}\times 1=\mathrm{b}\times 3\\ \Rightarrow \frac{\mathrm{a}}{\mathrm{b}}=\frac{1}{3}\end{array}$

$\Rightarrow \frac{{\mathrm{nFe}}^{2+}}{{\mathrm{nFe}}^{3+}}=\frac{2}{3}$