The Schulze-Hardy rule can be written in the approximate form: coagulation value is proportional to $1/z^6$, where $z$ is the valence of the coagulating ion. For a negative sol the coagulation value of $\mathrm{NaCl}$ is $50\mathrm{mmol}{\mathrm{L}}^{-1}$ of final mixture. What volume, in mL, of $0.010\mathrm{M}$ $\mathrm{AlCl}{}_3$ added to $250\mathrm{mL}$ of the sol just causes coagulation? Give two decimals.

Numerical answer type, answer in mL, tolerance 0.02.