# Understanding the Concepts of pH and pOH, and the Relationships Surrounding Concentration, Moles, Molarity, and Number of Molecules

An aqueous solution of an unknown substance has a pH of 3.0. If you have 0.25 L of the substance, how many (a) H+ ions, and (b) OH-ions, are present?

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...s the equation that defines the relationship between "pH" and "pOH". These equations are shown below.

pH = -log[H+]

pOH = -log[OH-]

pH + pOH = 14

From the question, we know that we have 250 mL of a pH 3.0 solution. Let's start with the first equation, and calculate the [H+]:

pH = -log[H+]
3.0 = -log[H+]
[H+]=10-3.0
[H+]=0.001 M

Now we know the concentration of [H+]....BUT....the question asks how many ions. Use the volume of solution to calculate the number of ions.

Concentration = # moles/volume
0.001 M = # moles/0.25 L
# moles = 0.00025

Only one more step to convert the number of moles to the number of ions...use Avogadro's number.