PQ ₂ is a sparingly soluble salt with solubility product K_ sp = 4 10⁻¹² in aqueous medium at some given temperature. It is observed that upon addition of a highly soluble salt RQ at the same temperature, the solubility of PQ ₂ drops by a factor of 100 . The concentration (in millimoles per litre) of added RQ in the solution is closest to:

Step-by-step solution

For the sparingly soluble salt PQ ₂ , the dissociation is given by: PQ ₂ P ²⁺ + 2 Q ^- Let the initial solubility in pure water be s₀ . The solubility product is: K_ sp = [ P ²⁺][ Q ^-]^2 = (s₀)(2s₀)^2 = 4s₀^3 Substituting the given value of K_ sp : 4s₀^3 = 4 10⁻¹² s₀ = 10⁻⁴ mol/L Upon adding the highly soluble salt RQ, the solubility drops by a factor of 100 . The new solubility s is: s = s₀ 100 = 10⁻⁴ 100 = 10⁻⁶ mol/L Let c be the concentration of the added salt RQ. The salt RQ dissociates completely to give Q ^-