(b)
$\mathrm{\Delta}x = i \times k_{f} \times m$
$7.10 \times 10^{- 3} = i \times 1.86 \times 0.001$
$i = 3.817$
$\because\ \alpha = \frac{i - 1}{n - 1}$
$\therefore 1 = \frac{3.817 - 1}{(x + 1) - 1}$
$x = 2.817 \approx 3$
$\therefore$ molecular formula of the compound is $K_{3}\lbrack Fe(CN)_{6}\rbrack$