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Strong or weak, acid or base ?

Universal indicator is useful for giving an estimated value for the pH of a substance

This is one of the tougher topics in year 13 chemistry.  In this post David deals with calculating the pH of strong acids and introduces some neat simulations.

 Assumed background knowledge

This topic is mostly about equilibria so it is important to understand that concept first. Before tackling this topic make sure you :

  • understand dynamic equilibria
  • can write and manipulate equilibrium expressions for a range of reactions.
  • can estimate the pH value of a solution using universal indicator 
  • know the pH values associated with : strong acids, weak acids, neutral solutions, weak alkalis and strong alkalis.

Strong or weak acid (GCSE revision) ?

Use this animation to remind yourself of the difference between strong and weak acids

When an acid molecule ionises completely when in solution in water, it is regarded as a strong acid. It is said to dissociate completely.

If we consider this ionisation to be an equilibrium - it would lie completely to the right.

At  A level you might say that the equilibrium constant (Kc) would be a very large value and not really useful.

When calculating pH values for strong acids we assume that the concentration of H+(aq) ions is equal to to the concentration of the acid

 Calculating pH for strong acids and alkalis

NOTE:  The aqueous form of H+ is often represented as H3O+ 

There are three modes in this  animation (below) by PHET

 :  1. Macro, 2. Micro, 3. My Solution. Have a go with each in turn...


pH is the negative logarithm of the concentration of hydrogen ions
log refers to base 10 logarithms or common logarithms

The common (base 10)  logarithm of x is the power to which the number 10 must be raised to obtain the value x. For example, the common logarithm of 10 is 1, the common logarithm of 100 is 2 and the common logarithm of 1000 is 3 

  • You should get used to calculating the log of a number.
  • Also practice using 10 x to anti log  
Calculate the concentration of hydrogen ions by raising 10 to the power of -pH.

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