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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...

#### SIMULATION

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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