The correct equation for the degree of association '$\mathrm{\alpha }$' of an associating solute, 'n' molecules of which undergoes association in solution, is

Consider following association reaction,
$$\begin{gathered} \mathrm{nA} \rightleftharpoons {\left(\mathrm{A}\right)}_{\mathrm{n}} \\ \mathrm{Initial} \mathrm{mole} 1 \mathrm{mole} 0 \\ \mathrm{Associated} \mathrm{change} -\mathrm{\alpha } \mathrm{mole} +\mathrm{\alpha }/\mathrm{n} \mathrm{mole} \\ \mathrm{Equilibrium} \mathrm{mole} (1-\mathrm{\alpha }) \mathrm{mole} \mathrm{\alpha }/\mathrm{n} \mathrm{mole} \end{gathered}$$
Total number of moles after association of 1 mole of A  = $(1-\mathrm{\alpha })+(\mathrm{\alpha }/\mathrm{n})$ mole
Total no. of moles = $1-\mathrm{\alpha }+\frac{\mathrm{\alpha }}{\mathrm{n}}$
van't Hofffactor (i) = $=\frac{\text{ Number of moles after association }}{\text{ Normal number of mole taken }}$
$=\frac{1-\mathrm{\alpha }+\frac{\mathrm{\alpha }}{\mathrm{n}}}{1}$