20mL of 0 . 5MNaCl is required to coagulate 200mL of As _ 2 S _ 3...

Coagulating value $=\dfrac{Millimoles of electrolyte}{Volume of sol in litre}$

$20ml$ of $0 . 5MNaCl$~contains $NaCl = 20 \times 0 . 5 = 10millimoles$

$200ml$ of ${As}_{2}{S}_{3}$ required $NaCl$ for the coagulation $= 10$~$millimoles$

$∴ 1000ml$ of ${As}_{2}{S}_{3}$ required $NaCl$ for the coagulation $= 10 \times 1000 / 200 = 50millimoles$.