Question

Calculate the concentration of F⁻ required to initiate precipitation of CaF₂ in a solution that is 0.010 M in Ca²⁺.

a) 0.005 M
b) 0.020 M
c) 0.010 M
d) 0.002 M

Answer 1

To initiate precipitation of CaF₂, a concentration of 0.020 M F⁻ is required.

Step-by-step explanation:

To calculate the concentration of F⁻ required to initiate precipitation of CaF₂, we need to use the solubility product constant (Ksp) of CaF₂. The equation for the dissociation of CaF₂ is CaF₂ (s) ⇌ Ca²⁺ (aq) + 2F¯ (aq). According to the principle of solubility product, the concentration of Ca²⁺ (0.010 M) is already given. The concentration of F⁻ required to initiate precipitation can be found by using the Ksp value of CaF₂.

From the equation, we can see that the molar ratio between Ca²⁺ and F⁻ is 1:2. So, for every 1 mole of Ca²⁺, we need 2 moles of F⁻ to form CaF₂. Since the concentration of Ca²⁺ is 0.010 M, the concentration of F⁻ required is 2 times that, which is 0.020 M. Therefore, the correct answer is option b) 0.020 M.