Answer:
a) Ksp = 7.9x10⁻¹⁰
b) Solubility is 6.31x10⁻⁶M
Explanation:
a) InF₃ in water produce:
InF₃ ⇄ In⁺³ + 3F⁻
And Ksp is defined as:
Ksp = [In⁺³] [F⁻]³
4.0x10⁻²g / 100mL of InF₃ are:
4.0x10⁻²g / 100mL ₓ (1mol / 172g) ₓ (100mL / 0.1L) = 2.3x10⁻³M InF₃. Thus:
[In⁺³] = 2.3x10⁻³M InF₃ × (1 mol In⁺³ / mol InF₃) = 2.3x10⁻³M In⁺³
[F⁻] = 2.3x10⁻³M InF₃ × (3 mol F⁻ / mol InF₃) = 7.0x10⁻³M F⁻
Replacing these values in Ksp formula:
Ksp = [2.3x10⁻³M In⁺³] × [7.0x10⁻³M F⁻]³ = 7.9x10⁻¹⁰
b) 0.05 moles of F⁻ produce solubility of InF₃ decrease to:
7.9x10⁻¹⁰ = [x] [0.05 + 3x]³
Where x are moles of In⁺³ produced from solid InF₃ and 3x are moles of F⁻ produced from the same source. That means x is solubility in mol / L
Solving from x:
x = -0.018 → False solution, there is no negative concentrations.
x = 6.31x10⁻⁶M → Right answer.
Thus, solubility is 6.31x10⁻⁶M
