pH (Edexcel International A Level Chemistry)

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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

Worked example

pH - Calculations

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 cmof 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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Sonny

Author: Sonny

Expertise: Chemistry

Sonny graduated from Imperial College London with a first-class degree in Biomedical Engineering. Turning from engineering to education, he has now been a science tutor working in the UK for several years. Sonny enjoys sharing his passion for science and producing engaging educational materials that help students reach their goals.