pH & pOH of Strong Acids & Bases (College Board AP® Chemistry) : Study Guide

Alexandra Brennan

Written by: Alexandra Brennan

Reviewed by: Stewart Hird

Updated on

pH & pOH of Strong Acids & Bases

Strong Acids

  • Most strong acids are monoprotic and fully ionize in aqueous solutions 

    • Examples include HCl (hydrochloric acid) and HNO3 (nitric acid) 

  • Due to the acid fully ionizing, there is a high concentration of  H+/H3O+ ions resulting in the solution having a low pH 

  • The H+/H3Oconcentration is equal to the molar concentration of the acid:

    • [H+ (aq)] = Mstrong acid

  • This can be used to calculate the pH using:

    • pH = -log10[H+ (aq)]

  • The position of the equilibrium is so far over to the right that you can represent the reaction as an irreversible reaction

Diagram to show the ionization of a strong acid

Diagram showing how strong acids dissociate

The diagram shows the complete dissociation of a strong acid in aqueous solution

Strong Bases

  • Strong bases, such as Group 1 metal hydroxides, completely dissociate in aqueous solutions

  • A large concentration of OH ions are present in the solution causing it to have a high pH 

  • Group 1 hydroxides produce one hydroxide ion per formula unit but others, such as Ca(OH)2 will produce more than one 

  • Therefore the concentration of hydroxide ions can be calculated using:

    • [OH-] = Mstrong base x number of OH- ions per mole

  • This can then be used to calculate pOH:

    • pOH = -log [OH]

  • pOH can then be used to calculate pH:

    • pH = 14 - pOH

  • The dissociation of a strong base  can be represented using the diagram below 

  • The position of the equilibrium is on the right and so the reaction is irreversible 

Diagram to show the dissociation of a strong base

Diagram showing how strong bases dissociate

The diagram shows the complete dissociation of a strong base in aqueous solution

Worked Example

1. Calculate the pH for a solution with a hydrogen ion concentration of 1.6 x 10-4 M.

2. Calculate the pH of a solution containing 0.00052 M Ba(OH)2.

Answers:

Answer 1:

  • The pH of the solution is:

    • pH = -log [H+]

    • pH = -log 1.6 x 10-4

    • pH = 3.80

Answer 2:

  • The pH of the solution is: 

    • [OH-] = 0.00052 x 2 = 0.00104

    • pOH = -log 0.00104 = 2.98

    • pH = 14 - 2.98 = 11.02

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Alexandra Brennan

Author: Alexandra Brennan

Expertise: Chemistry Content Creator

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.

Stewart Hird

Reviewer: Stewart Hird

Expertise: Chemistry Lead

Stewart has been an enthusiastic GCSE, IGCSE, A Level and IB teacher for more than 30 years in the UK as well as overseas, and has also been an examiner for IB and A Level. As a long-standing Head of Science, Stewart brings a wealth of experience to creating Topic Questions and revision materials for Save My Exams. Stewart specialises in Chemistry, but has also taught Physics and Environmental Systems and Societies.

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