You want to maintain pH = 7.0 for an enzyme-catalyzed reaction that will produce
ID: 879190 • Letter: Y
Question
You want to maintain pH = 7.0 for an enzyme-catalyzed reaction that will produce hydrogen ions along with the desired product. At equal concentrations, which weak acid, if any, will serve as the better buffer for the reaction: Acid A pKa = 6.5, Acid B pKa = 7.5
a. Acid A
b. both are equally effective
c. water is as good as either of the acids available
d. acid B
*** I get that answer A is correct but Wouldn't B (pKa = 7.5) be better because as more H+ gets released by the reaction the pH would go down closer to 7, while if A was used the pH would fall further away from 7 . Also I'm not sure what is at "equal concentrations" as in the question
Explanation / Answer
The correct answer is A. Weak acid A (pKa = 6.5).
- If we are working with a buffer, it means that we have a weak acid or base with its conjugated salt (conjugated basic or acid salt). In this case we have a weak acid with its conjugated basic salt.
- At equal concentrations: it means that you have a concentration of 0.1 M of Acid A and 0.1 M of Acid B.
So, which to choose and why?
Lets put an example. Here I am going to use the HH equation (Henderson-Hasselbach). This equation is used with Buffer solutions or common ion effect solutions.
Henderson-Hasselbach equation: pH = pKa + log ( base / acid )
1 -----> Let's say that we have 0.1 M of Acid A with 0.2 M of its conjugated basic salt. pKa = 6.5.
pH = pKa + log [base (conjugated salt) / acid (Acid A)]
pH = 6.5 + log ( 0.2 / 0.1 )
pH = 6.5 + 0.30
pH = 6.8
2 -------> Let's say that we have 0.1 M of Acid B with 0.2 M of its conjugated basic salt. pKa = 7.5
pH = pKa + log [base (conjugated salt) / acid (Acid B)]
pH = 7.5 + log ( 0.2 / 0.1 )
pH = 7.5 + 0.30
pH = 7.8
According to calculations, we see that with Acid A we obtain a pH close to 7, while with Acid B we obtain a pH > than 7.
So, if we choose a weak acid with a higher pKa (Acid B) the pH is going to be higher than 7.
If we choose a weak acid with a lower pKa (Acid A) the pH is going to maintain close to 7.
If you need more explanation or if you have any doubt please you could leave a comment.
Related Questions
drjack9650@gmail.com
Navigate
Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.