A solution of Ba(OH)2 in water has a pOH of 2.15. What is the barium ion concentration of this solution?

Question

[OH]=10^pOH

Ba(OH)2 --> Ba 2OH

Answer 1

A correction to your formula: [OH]=10^(-pOH)

Assuming that the dissociation of water does not play a role (it doesn't unless you are dealing with very small amounts), you would do the following.

Find out the concentration of OH in the solution:

[OH] = 10^(-2.15) = 0.00708 M OH

For every one Ba you have two OH. This means that the amount of Ba you have is HALF the amount of OH you have.

0.007079 / 2 = 0.00354 M Ba

(This is all assuming that Ba(OH)2 dissociates completely. If it has a dissociation constant, you must take that constant into consideration)