What is the Hydronium Ion Concentration in a Magnesium Hydroxide Solution?

[a.a]

Homework Statement

If 0.12 g of magnesium hydroxide Mg(OH)2 is dissolved in sufficient water to make 500. mL of solution, what is the hydronium ion concentration in the solution? (Assume that all of the magnesium hydroxide dissolves.)

Homework Equations

pH = -log [OH-]
pOH = -log [H3O+]

The Attempt at a Solution

I got 5 * 10^-12:

Mg(OH)2 --> Mg2+ + 2OH-
[OH-] = 2.058 * 10^-3 M

pOH = 2.6852
then pH = 14 - pOH = 11.31

[H3O+] = 10^-11.30
= 5*10^-12

is this the right method, is there an easier way?

 

[Borek]

Approach is correct, but the result is wrong. What is molar concentration of Mg(OH)₂?

 

[a.a]

M = .12/.5 = .24

 

[a.a]

got it.. thanks though

 

[a.a]

moles Mg(OH)2 = 2.058*10^-3

Mg(OH)2 --> Mg2+ + 2OH-
1 mol Mg(OH)2 ---> 2 mol OH-
2.058*10^-3 mol Mg(OH)2---> 2 * 2.058*10^-3 = 4.11*10^-3


thus [OH-] = 4.11*10^-3 M
pOH = 2.08
then pH = 14 - pOH = 11.9

[H3O+] = 10^-11.9
= 1.2*10^-12

:) is this rite?

 

[Borek]

2.058*10^-3 mol Mg(OH)2---> 2 * 2.058*10^-3 = 4.11*10^-3

This is number of moles, not concentration.