$$20$$ mL of $$0.5$$ M NaCl is required to coagulate $$200$$ mL of As$$_2$$S$$_3$$ solution in 2 hours. The coagulating value of NaCl is _____.

Coagulating value is defined as the millimoles of electrolyte required to cause coagulation of 1 L of sol.

Millimoles of NaCl used = 20 × 0.5 = 10 mmol

Volume of sol = 200 mL = 0.2 L

Coagulating value = $$\frac{10}{0.2} = 50$$ mmol/L

The coagulating value is $$\mathbf{50}$$.