Introduction

By definition, pH is the negative log of the hydronium ion (H+) concentration. The hydronium ion is simply the ion which make acids acidic. It is so reactive, because it easily bonds with other chemical to transform them into something with different properties. Likewise a basic solution is basic because of the hydroxide ion (OH-).

The hydronium and hydroxide consentrations are related to each other with the equation:

The relationship ends up being an equalibrium system where as the concentration of hydronium drops the concentration of hydroxide will rise to compensate. This means that as soon as one can determine the concentration of either of these ions, one can calculate pH. In pure undisturbed water this equalibrium will exist at a pH of 7 with the same concentration of each ion.

It is important to note that the lower the pH number the greater the acidity and also that the difference between a three and a four is that the 3 is ten times more acidic than the four.

Finding pH

To find what the pH of a certain solution via calculations requires a few steps. First one must determine that the compound containing either a hydronium ion or hydroxide ion will completely disassociate in water (otherwise the problem becomes much more complex). All strong acids and bases completely dissociate in water so they are good examples. Then it simply a matter of dividing and then taking the log.

If you were given a 100mL of distilled water and you then added 0.2 moles of HNO3, what would the pH and pOH of the resulting solution be?

First determine that nitric acid is strong, thus determining that all hydrogens will be released. Now determine the molarity of H+ by doing: .2mole / .1L = 2 molar (H+). Now take the negative log of 2 which gives you -.3. Now this is a resonalble answer because the pH scale is not bounded to positive numbers but it should be quite clear that this is a very acidic solution. To find the pOH simply add the opposite to 14 thus giving you 14.3.

Examples

To aquaint you better with the pH scale here is a chart of some common items with their pH: