Question 51

[dnpgr16513]

Can you please go into detail with the relationship between pH vs. pKa? This is a confusing concept to me. Also, how do we know to refer back to the equation in the passage?

thanks.

[jellywing_2058]

The pKa is a measure of the Ka of an acid molecule (not the solution) or the dissociation of the acid itself. Moreover, the smaller the pKa, the stronger the acid (more dissociation occurs as it is the log of the Ka which is the dissociation value of an acid).

The pH is a measure of the H+ concentration or the acidic property of a solution. It is not the acid molecule dissociation potential (or in this case the indicator itself in solution).

The relationship is that the pH numerical value is descriptive of the acidic property of “the solution itself” whereas the pKa is a measure of the potential acidic property of the “acidic indicator molecule” itself.

The smaller the pKa (or larger the Ka), the stronger the H+ dissociation property of the molecule. Essentially, if the pH of the solution is much lower than the pKa of the indicator, the solution is very acidic with many released H+ surrounding the indicator. As such, the indicator is in its protonated stage as there are lots of free H+ within the solution.

There is no “real absolute direct” relationship in this case and one is just to compare the pH of the solution versus the pKa of the indicator molecule itself to see which state the indicator will be in within the respective solution. In this case, the solution is very acidic with a pH of 2.0.

Note however, for buffers, a relationship between pH and pKa (as per your question) does exist and is as follows; pH = pKa + log {[conjugate base]/[acid]} which is otherwise known as the Henderson-Hasselbalch equation, which in this case is not directly applicable.