# Homework Help: Concentration of Hydronium

1. Nov 1, 2008

### a.a

1. The problem statement, all variables and given/known data

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.)

2. Relevant equations

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

3. 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?

2. Nov 1, 2008

### Staff: Mentor

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

Last edited: Nov 1, 2008
3. Nov 3, 2008

### a.a

M = .12/.5 = .24

4. Nov 3, 2008

### a.a

got it.. thanks though

5. Nov 3, 2008

### 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?

6. Nov 3, 2008

### Staff: Mentor

This is number of moles, not concentration.