Inorganic and Applied Chemistry

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Inorganic and Applied Chemistry

Example 5- A:

pH calculation in a strong acid (hydrochloric acid)

A solution consists of 0.5 M HCL. We know that HCl is a strong acid which is why it is reasonable to

assume that the acid is completely dissociated meaning that the following reaction is completely turned to

the right:

HCl(aq) + H 2 O(l)  H 3 O+(aq) + Cl-(aq)

We neglect as it is often the case the contribution to the concentration of H 3 O+ from autoprotolysis of

water and assume that all HCl molecules are dissociated which is why:

[H 3 O+] = 0,5 M

Now the pH value is easily achieved as:

pH logH 3 O log 0 , 5  0 , 3

5.2.2 Calculation of pH in weak acid solutions

As mentioned earlier it is necessary to account for the equilibrium in the case with weak acids. As the

following example illustrate one has to proceed in steps when pH is to be calculated in weak acid solutions.

Acids and bases

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