100 ml of 0.0018% (w/v) solution of Cl⁻ ion was the minimum concentration of Cl⁻ required to precipitate a negative sol in one h. The coagulating value of Cl⁻ ion is _________ (Nearest integer)

Solution

Chemistry ma’am · 3 steps

Coagulating value is millimoles per litre, so turn the percentage into grams per litre first.

1. What is asked. The coagulating value of an ion is the minimum amount of that ion, in millimoles per litre, needed to coagulate the sol.
2. Read the percentage. A 0.0018% (w/v) solution holds 0.0018 g of Cl⁻ in every 100 mL. Scaled up to one litre, that is 0.018 g L⁻¹.

   (0.018 g L⁻¹)/(35.5 g mol⁻¹) = 5.07 × 10⁻⁴ mol L⁻¹ = 0.507 mmol L⁻¹

3. So. The coagulating value of Cl⁻ is about 0.507 mmol L⁻¹, which rounds to the nearest integer 1.

The common slip: 0. Leaving the amount per 100 mL instead of per litre gives 0.0507 mmol, which rounds to 0, not 1.

The idea to keep. Coagulating value always means millimoles of the ion per litre of sol. Convert w/v percent to grams per litre, divide by molar mass, then multiply by 1000.

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