Solubility

Question 1

What is the molar solubility of ferrous (II) hydroxide in water at 25°C?

Answer 1

2.1 x 10-5
The passage states that the Ksp of ferrous (II) hydroxide is 3.2 x 10-14. When Ksp is known, we can determine molar solubility from the dissociation reaction.

Fe(OH)2 (aq) + H2O (l) → Fe2+(aq) + 2 OH- (aq)

Note that Fe(OH)2 dissociates into three ions (one Fe2+ and 2 OH-). Given this 2:1 ratio, Ksp = [Fe2+][OH-]2 = [x][2x]2 = 4x3, where x represents the molar solubility.

Next, we must divide Ksp by 4, then take its cube root to solve for x. Dividing 3.2 by 4 is more difficult than dividing 32 by 4, so we can manipulate scientific notation and rewrite Ksp in an easier format.

Ksp = 4x3 = 3.2 x 10-14 = 32 x 10-15

8 x 10-15 = x3

2 x 10-5 = x

Question 2

True or False: Ksp is not impacted by concentration

Answer 2

True, Ksp abides by the same rules as Keq