Question

What is the solubility in moles/liter for iron(III) hydroxide at 25 oC given a Ksp value of 2.0 x 10-39. Write using scientific notation and use 1 or 2 decimal places (even though this is strictly incorrect!)

Answer 1

Answer:

9.28 × 10⁻¹¹ mol/L

Explanation:

Let's consider the solution of iron(III) hydroxide.

Fe(OH)₃(s) ⇄ Fe³⁺(aq) + 3 OH⁻(aq)

We can relate the solubility (S) of the hydroxide with the solubility product (Ksp) using an ICE chart.

      Fe(OH)₃(s) ⇄ Fe³⁺(aq) + 3 OH⁻(aq)

I                              0                  0

C                            +S               +3S

E                              S                 3S

The solubility product is:

Ksp = [Fe³⁺] × [OH⁻]³ = S × (3S)³ = 27 S⁴

$S=\sqrt[4]{\frac{Ksp}{27} } = \sqrt[4]{\frac{2.0 \times 10^{-39} }{27} } = 9.28 \times 10^{-11} mol/L$

Answer 2

Answer:

S = 9.28 E-11 M

Explanation:

• Fe(OH)3 ↔ Fe3+  +  3OH-

          S                 S            3S

∴ Ksp Fe(OH)3 = 2.0 E-39

⇒ Ksp = [Fe3+]*[OH-]³ = (S)*(3S)³ = 27(S)∧4

⇒ 27(S)∧4 = 2.0 E-39

⇒ (S)∧4 = 7.407 E-41

⇒ S = (7.407 E-41)∧(1/4)

⇒ S = 9.28 E-11 M