Sample 1Medium
A beaker shows solid calcium fluoride at the bottom in contact with its saturated aqueous solution at 298 K, with double arrows between the solid and the Ca²⁺(aq) and F⁻(aq) ions above it, marking a dynamic equilibrium. The molar solubility of CaF2 here is 2.0 × 10⁻⁴ mol L⁻¹. What is its solubility product?
Show the answer
The answer is B.
CaF2(s) ⇌ Ca²⁺(aq) + 2F⁻(aq), so [Ca²⁺] = s and [F⁻] = 2s. Ksp = s(2s)² = 4s³ = 4 × (2.0 × 10⁻⁴)³ = 3.2 × 10⁻¹¹ mol³ L⁻³.
- A.
This multiplies s by 2s without squaring the fluoride concentration.
- C.
s² applies to a 1:1 salt; CaF2 gives one Ca²⁺ and two F⁻ ions.
- D.
This is s³ without the factor 4 that comes from (2s)².
NCERT: Class 11 Chemistry, Chapter 6