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The pOH Scale (HL) (HL IB Chemistry)

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The pOH Scale

pH

  • The acidity of an aqueous solution depends on the number of H+ ions in the solution
  • pH is defined as:

pH = -log10 [H+]

    • Where [H+] is the concentration of H+ 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
    • For example, pH 5 is 10 times more acidic than pH 6
    • This means that each value is 10 times the value below it
  • pH values are usually given to 2 decimal places

pOH

  • The basicity of an aqueous solution depends on the number of hydroxide ions, OH-, in the solution
  • pOH is defined as:

pOH = -log [OH-]

    • Where [OH-] is the concentration of hydroxide ions in mol dm–3
  • Similarly, the concentration of OH- of a solution can be calculated if the pH is known by rearranging the above equation to:

[OH-] = 10-pOH

  • If you are given the concentration of a basic solution and need to find the pH, this can be done by:

[H+] = Kw / [OH-]

  • Alternatively, if you are given the [OH-] and calculate the pOH, the pH can be found by:

pH = 14 - pOH

  • As we can see, pH and pOH are interlinked and at all temperatures, pH + pOH = pKw

Relationship between H+, OH–, pH and pOH

Diagram to show the relationship between H+, pH, OH- and pOH

To make a conversion, follow the arrow and equation given, so to convert OH– (aq) to pOH use pOH = -log10[OH–] 

Worked example

pH and H+ calculations

  1. Find the pH when the hydrogen ion concentration is 1.60 x 10-4 mol dm-3
  2. Find the hydrogen ion concentration when the pH is 3.10

 

Answers:

  1. The pH of the solution is:
    • pH = -log [H+]
    • pH = -log 1.6 x 10-4
    • pH = 3.80

  2. The hydrogen concentration can be calculated by rearranging the equation for pH
    • pH = -log [H+]
    • [H+] = 10-pH
    • [H+] = 10-3.10
    • [H+] = 7.94 x 10-4 mol dm-3  

Worked example

pH calculations of a strong alkali

  1. Calculate the pH of 0.15 mol dm-3 sodium hydroxide, NaOH
  2. Calculate the hydroxide concentration of a solution of sodium hydroxide when the pH is 10.50

 

Answers:

  • Sodium hydroxide is a strong base which ionises as follows:

NaOH (aq) → Na+ (aq) + OH– (aq) 

  1. The pH of the solution is:
    • [H+] = Kw  ÷ [OH–]
    • [H+] = (1 x 10-14) ÷ 0.15 = 6.66 x 10-14
    • pH = -log 6.66 x 10-14  = 13.17pH = -log[H+] 

  2. To calculate the hydroxide concentration of a solution of sodium hydroxide when the pH is 10.50:
    • Step 1: Calculate hydrogen concentration by rearranging the equation for pH
      • pH = -log[H+]
      • [H+] = 10-10.50
      • [H+] = 3.16 x 10-11 mol dm-3[H+] = 10-pH

    • Step 2: Rearrange the ionic product of water  to find the concentration of hydroxide ions
      • Kw = [H+] [OH–]
      • [OH–] = Kw  ÷  [H+]

    • Step 3: Substitute the values into the expression to find the concentration of hydroxide ions
      • Since Kw is 1.00 x 10-14 
      • [OH–] = (1 x 10-14)  ÷  (3.16 x 10-11)
      • [OH–] = 3.16 x 10-4 mol dm-3
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Philippa

Author: Philippa

Philippa has worked as a GCSE and A level chemistry teacher and tutor for over thirteen years. She studied chemistry and sport science at Loughborough University graduating in 2007 having also completed her PGCE in science. Throughout her time as a teacher she was incharge of a boarding house for five years and coached many teams in a variety of sports. When not producing resources with the chemistry team, Philippa enjoys being active outside with her young family and is a very keen gardener.