pH is a unitless number used to convey the acidity of a substance without having to deal with numbers in scientific notation. You must use an extra significant digit when calculating it (and drop a digit when you are going from pH to concentration). You can go between pH and concentration with these equivalent formulae:
pH=−log[H+]⇔[H+]=10−pH.
The pH value tells us about the acidity of a solution:
pH=7⇒ neutral
pH<7⇒ acidic
pH>7⇒ basic
pOH is just like pH, except it uses the hydroxide ion concentration:
pOH=−log[OH−]⇔[OH−]=10−pOH.
If you have one of pH and pOH, you can easily find the other (at SATP):
pH+pOH=14.
Example
Calculate the concentrations of H+(aq) and OH−(aq) and the values of pH and pOH for a 0.042 mol/L H2SO4(aq) solution.
H2SO4(aq) is a strong acid, so finding the H+(aq) concentration is trivial:
[H+]=2[H2SO4]=2(0.042mol/L)=0.084mol/L.
Now we can use this to calculate pH. Remembering to keep an extra significant digit, we have
pH=−log[H+]=−log(0.084mol/L)=1.08,
and then we can calculate pOH with
pOH=14−pH=14−1.08=12.9.
From that we can get the hydroxide ion concentration, remembering to drop the extra significant digit: