Understanding pH, pOH, and the Strength of Acids and Bases
Dot-Point 2: calculate pH, pOH, hydrogen ion concentration ([H+ ]) and hydroxide ion concentration ([OH– ]) for a range of solutions (ACSCH102)
The pH scale is an essential tool for determining the acidity or basicity of a solution. By understanding the relationship between hydrogen ion concentration [H+] and hydroxide ion concentration [OH-] we can quantify the strength of acids and bases.
What is pH?
The pH of a solution measures its acidity, based on the concentration of hydrogen ions ([H+][H^+], also called hydronium ions [H3O+]. The pH scale ranges from 0 (highly acidic) to 14 (highly basic), with 7 being neutral.
The formula for pH is:
pH = -log[H⁺]
For example:
If [H+] is 0.001 M, the pH is:
pH = -log(0.001) = 3.0
Calculating Hydrogen Ion Concentration:
For example:
If the pH of a solution is 4.5, the hydrogen ion concentration is:
[H⁺] = 10^(-4.5) = 3.16 × 10⁻⁵ M
What is pOH?
The pOH measures the basicity of a solution, based on the concentration of hydroxide ions [OH−][OH^-]. The formula is similar to pH:
The formula for pH is:
pOH = -log[OH⁻]
Just as with pH, if [OH−][OH^-] is known, the value can be calculated as:
[OH⁻] = 10^(-pOH)
Relating pH and pOH
The pH and pOH scales are interconnected through the self-ionization of water. In pure water at 25°C:
[H⁺] × [OH⁻] = 1.0 × 10⁻¹⁴
This is represented by the equation:
pH + pOH = 14
This means if you know one value (either pH or pOH), you can easily calculate the other. For example:
If the pH is 5.0,
then pOH = 14 - 5.0 = 9.0.
Acidic & Basic Solutions
The concentration of hydrogen and hydroxide ions determines whether a solution is acidic or basic:
Acidic solutions:
[H+]>10−7
[OH−]<10−7
pH < 7
Basic solutions:
[H+]<10−7
[OH−]>10−7
pH > 7
