The Van't Hoff factor for a 0.1 M Al₂(SO₄)₃ solution is 4.20. The degree of dissociation is

$$\begin{gathered} {\mathrm{Al}}_2{\left({\mathrm{SO}}_4\right)}_3 \rightleftharpoons 2{\mathrm{AI}}^{3+} + 3\mathrm{S}{{\mathrm{O}}_4}^{2-} \\ 1 0 0 \\ 1-\mathrm{\alpha } 2\mathrm{\alpha } 3\mathrm{\alpha } \\ \mathrm{i}=\frac{1-\mathrm{\alpha }+2\mathrm{\alpha }+3\mathrm{\alpha }}{1}=1+4\mathrm{\alpha } \\ 4.2=1+4\mathrm{\alpha } \\ \mathrm{\alpha }=0.8 \end{gathered}$$

Therefore, the degree of dissociation is 80%.