Expert: Dr. Jeffery Raymond - 6/28/2010

Question
I have no idea how to solve these 3 chemistry problems.  Would you please help?  Thanks you in advance for your time!

What would the pOH of a solution be if the [H+] = 3.00 x 10-5 moles/liter?

If a solution had an [OH-] = 3.00 x 10-10 moles/liter, what would be the pH value of this solution?

If the pH value was 3.6, what is the [H+]?

Answer
We are not supposed to do homework problems for students.  That said, I will give you an example of each problem and solve it so that you can see how these *types* of problems are tackled. This is something that you will probably catch on to pretty quickly once you see it a couple times.

first, there are three equations that you will need to know.

1.  pH = - log [H+] = log (1/[H+])
    a side effect of this is that...
2.  10^(pH) = 1/[H+] and 10^(-pH) = [H+]

and the relationship between pOH and pH can be written like this:
3.  pOH = 14 - pH = - log [OH-]

This means that if we know the pOH, the pH, or the molarity of H+ or OH- then we know all the rest.

Sample questions
1.  what would the pOH be if the [H+] = 8x10-7 mol/L?

pH = - log (8x10-7) = 6.09
then
pOH = 14 - 6.09 = 7.91

2.  If a solution had an [OH-] = 5x10-3 mol/L, what would be the pH?

pOH = - log (5x10-3) = 2.30 = 14 - pH
then
pH = 14 - 2.3 = 11.7

3.  If the pH value was 4.7, what is the [H+]?

10^(-4.7) = [H+] = 2.0*10^-5 mol/L

The scientific calc. on your computer can handle all of these if you don't have scientific calculator.
This should get you off on the right foot.
Take care!