pH (Edexcel International A Level Chemistry): Revision Note
pH - Introduction
The acidity of an aqueous solution depends on the number of H+ ions in solution
The pH is defined as:
pH = -log[H+]
where [H+] is the concentration of hydrogen ions in mol dm–3
Similarly, the concentration of H+ of a solution can be calculated if the pH is known by rearranging the above equation to:
[H+] = 10-pH
The pH scale is a logarithmic scale with base 10
This means that each value is 10 times the value below it. For example, pH 5 is 10 times more acidic than pH 6.
pH values are usually given to 2 decimal places
The relationship between concentration is easily seen on the following table
pH & [H+] Table
![pH and [H+] Table, downloadable IB Chemistry revision notes](https://cdn.savemyexams.com/cdn-cgi/image/f=auto,width=3840/https://cdn.savemyexams.com/uploads/2021/06/8.1.6-pH-and-H-Table.png)
pH - Calculations
Worked Example
pH and H+ calculations
Question 1: Find the pH when the hydrogen concentration is 1.60 x 10-4 mol dm-3
Question 2: Find the hydrogen concentration when the pH is 3.10
Answer
Answer 1:
The pH of the solution is:
pH = -log[H+]
= -log 1.6 x 10-4
= 3.80
Answer 2:
The hydrogen concentration can be calculated by rearranging the equation for pH
pH = -log[H+]
[H+] = 10-pH
= 10-3.10
= 7.94 x 10-4 mol dm-3
Worked Example
Powers of 10
10.0 cm3 of an aqueous solution of an acid of pH = 1.0 is mixed with 990.0 cm3 of distilled water. What is the pH of the final solution?
A. 1
B. 2
C. 3
D. 10
Answer
The correct option is C.
The total volume after dilution is 1000.0 cm3 so the concentration of H+ has been reduced by a factor of 100 or 10-2, which means an increase of 2 pH units
The final solution is therefore pH 3
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