$20 \mathrm{~mL}$ of $0.5 \mathrm{M} ~\mathrm{NaCl}$ is required to coagulate $200 \mathrm{~mL}$ of $\mathrm{As}_{2} \mathrm{S}_{3}$ solution in 2 hours. The coagulating value of $\mathrm{NaCl}$ is ____________ .