pH & pOH
- The pH scale is a numerical scale that shows how acidic or alkaline a solution is
- The values on the pH scale go from 0 - 14 (extremely acidic substances have values of below 0)
- All acids have pH values below 7, all alkalis have pH values above 7
- The lower the pH then the more acidic the solution is
- The higher the pH then the more alkaline the solution is
The pH scale
The pH scale showing acidity, neutrality and alkalinity
pH of acids
- Acidic solutions (strong or weak) always have more H3O+ than OH- ions
- Since the concentration of H3O+ is always greater than the concentration of OH- ions, [H3O+] is always greater than 10-7 M
- Using the pH formula, this means that the pH of acidic solutions is always below 7
- The higher the [H3O+] of the acid, the lower the pH
pH of bases
- Basic solutions (strong or weak) always have more OH- than H3O+ ions
- Since the concentration of OH- is always greater than the concentration of H3O+ ions, [H3O+] is always smaller than 10-7 M
- Using the pH formula, this means that the pH of basic solutions is always above 7
- The higher the [OH–] of the base, the higher the pH
pH of water
- Water at 298K has equal amounts of OH- and H3O+ ions with concentrations of 10-7 M
- To calculate the pH of water, the following formula should be used:
pH = – log10[H3O+ (aq)]
[H3O+ (aq)] = concentration of H+/H3O+ ions
pH = -log (10-7) = 7
- Thus, water has a pH of 7 at 298 K
pH
- The acidity of an aqueous solution depends on the number of H3O+ ions in the solution
- pH is defined as:
pH = -log10 [H3O+]
-
- Where [H3O+] is the concentration of H3O+ ions in M
- Similarly, the concentration of H3O+ of a solution can be calculated if the pH is known by rearranging the above equation to:
[H3O+] = 10-pH
- The pH scale is a logarithmic scale with base 10
- For example, pH 5 is 10 times more acidic than pH 6
- This means that each value is 10 times the value below it
- pH values are usually given to 2 decimal places
pH & [H3O+] Table
[H3O+] | Scientific notation | pH |
1.0 | 100 | 0 |
0.1 | 10–1 | 1 |
0.01 | 10–2 | 2 |
0.001 | 10–3 | 3 |
0.0001 | 10–4 | 4 |
- | 10–x | x |
pOH
- The basicity of an aqueous solution depends on the number of hydroxide ions, OH-, in the solution
- pOH is defined as:
pOH = -log [OH–]
-
- Where [OH-] is the concentration of hydroxide ions in M
- Similarly, the concentration of OH- of a solution can be calculated if the pH is known by rearranging the above equation to:
[OH–] = 10-pOH
- If you are given the concentration of a basic solution and need to find the pH, this can be done by:
[H3O+] =
- Alternatively, if you are given the [OH-] and calculate the pOH, the pH can be found by:
pH = 14- pOH
- As we can see, pH and pOH are interlinked and at all temperatures, pH + pOH = pKw
Exam Tip
Hydronium ion and H3O+ (aq) ideally should be written, but H+ (aq) is also accepted on the AP Exam