1. Calculating pH
SCH3U/4C
Ms. richardson

2. The Power of Hydrogen pH stands for the “power of hydrogen”
It is a way of indicating the concentration of hydrogen ions present in a solution.
The scale is logarithmic, based on factors of 10.

3. The pH Scale The pH scale ranges from 0-14.
A pH of 7 means that the substance is neutral.
The pH scale is a logarithmic function of the concentration of H+
As a result, each whole pH value below 7 is ten times more acidic than the next higher value.
The same holds true for pH values above 7, each of which is ten times more alkaline (basic)

4. pH and Concentration
Pure water has a hydrogen ion concentration of about 1 X 10-7 mol/L.
[H+] = 1 X 10-7 mol/L.
In an acidic solution, [H+] > 1 X 10-7 mol/l
In a basic solution, [H+] < 1 X 10-7 mol/l

5. Calculating pH
To calculate the pH of an aqueous solution you need to know the concentration of the hydronium ion in moles per liter (molarity).  The pH is then calculated using the expression:
pH = - log [H3O+]

6. Example Find the pH of a 0.0025 M HCl solution.
(Remember - HCl fully ionizes in solution)
pH  =  - log (0.0025)
= - ( )
= 2.60

7. Calculating H+ Concentration
The hydronium ion concentration can be found from the pH by the reverse of the mathematical operation employed to find the pH.
[H3O+] = 10-pH  or  [H3O+] = antilog (- pH)

8. Example
What is the hydronium ion concentration in a solution that has a pH of 8.34?
8.34 = - log [H3O+]  = log [H3O+]  [H3O+] =  
= 4.57 x 10-9 M