Question

Calculate pH of a solution containing 0.1M HA (${K_a} = {10^{ - 5}}$) and 0.1M HCl?

Answer 1

The chemicals have different natures - they can be neutral or acidic or basic. These characteristics depend primarily on how many ${H^ + }$ or $O{H^ - }$ ions a particular chemical is releasing in its aqueous solution. Chemicals that release ${H^ + }$ ions are acidic and those which release $O{H^ - }$ ions are basic. So, pH plays a very important role in determining the acidic or basic strength of a chemical.

Complete answer: pH is defined as the negative logarithm (base 10) of hydrogen ion concentration or it can be termed as potential of hydrogen ion.
The pH scale is from 0 to 14.
Acid pH < 7
Base pH > 7
Neutral pH=7
To calculate pH of a solution, from the definition of pH we have,
$pH = - \log [{H^ + }]$
The pH of the solution is the measure of the hydrogen ion concentration which in turn is the measure of its acidity.
$HA \to {H^ + } + {A^ - }$
0.1M
$HCl \to {H^ + } + C{l^ - }$
0.1M
The concentration of both solutions is 0.1M.
When the hydrogen ion is in excess, the solution is considered as acidic and when the hydrogen ion is less compared to hydroxyl ion then the solution is considered to be basic.
Now let us calculate the pH of the given solution.
Rewriting the pH equation,
$pH = - \log [{H^ + }]$
$\Rightarrow pH = - \log [0.1]$
$\Rightarrow pH = - \log [{10^{ - 1}}]$
On simplification,
$pH = 1$.
Therefore the pH of the given solution containing 0.1M HA and 0.1M HCl is 1 .

Note:
The solution is said to be a strong acid when the pH is 0 or less than 5 and the solution is said to be a weak acid which has pH around 5 to just below 7 and this weak acid even sparsely ionizes.