Effect of Hydrogen Ion Concentration on pH (OCR Gateway GCSE Chemistry: Combined Science)

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Effect of Hydrogen Ion Concentration on pH

  • pH is a measure of the concentration of H+ ions in solution
  • A concentrated solution of either an acid or a base is one that contains a high number of acid or base molecules per dm3 of solution so would produce pH values below 4 and above 10
  • A dilute acid or base solution is therefore one that has much fewer acid or base molecules per dm3 of solution, hence the pH value would lie between 5 and 9
  • The pH scale is logarithmic, meaning that each change of 1 on the scale represents a change in concentration by a factor of 10
  • Therefore an acid with a pH of 3 has ten times the concentration of H+ ions than an acid of pH 4
  • An acid with a pH of 2 has 10 x 10 = 100 times the concentration of H+ ions than an acid with a pH of 4
  • From this we can summarize that for two acids of equal concentration, where one is strong and the other is weak, the strong acid will have a lower pH due to its capacity to dissociate more and hence put more H+ ions into solution than the weak acid

Titration Curves

  • A pH titration curve shows the effect of changing hydrogen ion concentration on pH 
  • The titration curve on the left demonstrates what happens to the pH when a base is added to an acid 
    • The pH increases rapidly due to a decrease in the concentration of hydrogen ions
  • The titration curve on the right demonstrates what happens to pH when an acid is added to a base 
    • The pH decreases rapidly due to an increase in the concentration of hydrogen ions 

pH curves - different versions, downloadable AS & A Level Chemistry revision notes

Titration curves demonstrating the effect of changing pH when a base is added to an acid and vice versa 

Examiner Tip

Acid strength indicates the proportion of acid molecules that dissociate while concentration is a measure of how much acid there is per unit volume of water.

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Alexandra

Author: Alexandra

Expertise: Chemistry

Alex studied Biochemistry at Newcastle University before embarking upon a career in teaching. With nearly 10 years of teaching experience, Alex has had several roles including Chemistry/Science Teacher, Head of Science and Examiner for AQA and Edexcel. Alex’s passion for creating engaging content that enables students to succeed in exams drove her to pursue a career outside of the classroom at SME.