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

S = 9.28 E-11 M

Step-by-step 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

Answer 2

9.28 × 10⁻¹¹ mol/L

Step-by-step 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]{(Ksp)/(27) } = \sqrt[4]{(2.0 * 10^(-39) )/(27) } = 9.28 * 10^(-11) mol/L`