A measure of the concentration of hydronium ions (H3O+) in a solution is pH, and a measure of the concentration of hydroxide ions is pOH. The pH scale ranges from 1 to 14, with 1 being most acidic and 14 being most basic.
The pH of a substance is a measure of the concentration of H+ (or H3O+) ions in solution. The relationship is logarithmic (base 10): the pH value itself is the value of the exponent, x, when the concentration is expressed as . In other words, in a substance that has a pH of 5, the concentration of H+ ions is . The pOH of a substance is a measure of the concentration of OH– ions in solution. The total of the exponents must equal 14, so in a substance with a pH of 5, the pOH must be 9. When the two values are compared, and , the concentration of H+ is greater than that of OH– by four orders of magnitude. When [H+] is greater than [OH–], the solution is acidic. When the opposite is true, the solution is basic.
The following equation relates pH to the concentration of H+
For example, calculating [H3
] of 0.20 M acetic acid (CH3
COOH) at equilibrium
gives a concentration of
. Substitute x
] and [CH3
] in the equation for the equilibrium constant
, and substitute 0.20 for [CH3
Substitute the known value of Ka
, and rearrange the equation to solve for x
The value of x
equals the concentration of the hydronium ion.
The negative logarithm is the pH of the solution.
Thus the pH can be calculated knowing only Ka
and the concentration of an acid.
Because pH and pOH must add up to 14, the pOH of the solution can also be determined:
The values of pH and pOH can also be used to calculate [H3
] or [OH–