20 mL of 0.5 M NaCl is required to coagulate 200 mL of As₂ S₃ solution in 2 hours. The coagulating value of NaCl is ____________ .

Solution

Chemistry ma’am · 3 steps

Coagulating value is millimoles per litre of sol, so find the millimoles and scale 200 mL up to a litre.

1. What it means. The coagulating (flocculation) value is the minimum amount of electrolyte, in millimoles, needed to coagulate 1 litre of sol in 2 hours. The 2 hours in the question is the standard time, so no time correction is needed.
2. Millimoles of NaCl. Millimoles = volume in mL × molarity, so 20 × 0.5 = 10 millimoles of NaCl are used.

   Coagulating value = (10 mmol)/(200 mL) × 1000 mL = 50 mmol L⁻¹

3. So. The 10 millimoles coagulate 200 mL of sol, so 1000 mL would need five times as much. The coagulating value of NaCl is 50.

The common slip: 10. Stopping at the millimoles used gives 10, not 50. That is the amount for 200 mL of sol, so it still has to be scaled up to 1 litre.

The idea to keep. Coagulating value is always millimoles of electrolyte per litre of sol. Find the millimoles used, then scale to 1000 mL of the sol itself.

Answer from the NTA final answer key. Solution by PrepWiser.