Question

How would you use the Henderson-Hasselbalch equation to calculate the pH of a solution that is $0.120M$ in $HClO$ and $0.185M$ in the $KClO$ ?

Answer 1

Hint : The Henderson-Hasselbalch equation is essential to estimate the pH of a buffer solution and also for estimating the equilibrium pH of an acid base reaction. It determines the pH using the numerical value of the amount of acid and conjugate base required to form a buffer solution and also the acid dissociation constant of the acid.

Complete Step By Step Answer:
The Henderson-Hasselbalch formula to calculate the pH is:
$pH = p{K_a} + \log \left( {\dfrac{{Base}}{{Acid}}} \right)$
As we can see, in this case $HClO$ acts as a weak acid and $KClO$ is the conjugate base.
Molar concentration of $HClO = 0.120M$
Molar concentration of $KClO = 0.185M$
The dissociation constant of $HClO = 3.5 \times {10^{ - 8}}$
Now, substituting these values to the equation, we get,
$pH = - \log (3.5 \times {10^{ - 8}}) + \log \left( {\dfrac{{0.185}}{{0.120}}} \right) \\\ pH = 7.46 + \log \left( {1.542} \right) \\\ pH = 7.46 + 0.188 \\\ pH = 7.65 \\\$
Hence, The pH of a solution is 7.56.

Note :
The Henderson-Hasselbalch formula works only when the solvent is water and it is present in a large proportion compared to the conjugate base. The approximation is applicable only when the following conditions are met:
1. $- 1 < \log (\dfrac{{[A - ]}}{{[HA]}}) < 1$
2. The molarity of the buffers should be hundred times more than that of the acid dissociation constant.
3. The equation fails if the dissociation constant doesn’t lie between $5$ and $9$ . It will provide us with inaccurate results. Hence, for best results the dissociation should always lie between $5$ and $9$ .