How do you calculate the ph from the [oh-] ?

POH is defined as the hydroxylonic potential of a solution, that is, the amount of hydroxides (OH-) dispersed in a certain volume of solvent (such as water).

To calculate the pOH of a solution, we use the following mathematical expressions:

First:

pOH = - log [OH-]

log: is the base 10 logarithm.

[OH-]: concentration in quantity of matter (molarity) of hydroxides.

Second:

10-pOH = [OH-]

This calculation almost always depends on the value of the OH- concentration, but knowing the pH (hydrogen potential, that is, the amount of H + cations) can also be useful. This is possible because, according to the theory of ionic water balance, the concentrations of H + and OH- in a neutral medium, at 25 oC, are always 10-7 mol / L. For this reason, we can say that:

pH + pOH = 14

NOTE: Water is capable of self-ionization and produces hydronium and hydroxide ions in the medium, as shown in the following equation:

H2O → H + + OH-

If it is necessary to use pH in an exercise on calculating pOH, just use the mathematical expressions referring to it, which are:

pH = - log [H +]

or

10-pH = [H +]

Hope this helps!