"A substance has a pKa value of 7. At pH 4, approximately what percentage of this substance is in protonated form?"
Hello everyone,
Chemistry is one of my biggest weaknesses, and here's a question I can't find an answer to anywhere. From the information provided, I can conclude that this is a weak acid. Since the pH is below 7, the solution is definitely acidic, but not strongly acidic. Using the formula pH = 1/2 * (pKa – log(c)) and rearranging, I can also calculate the acid concentration of 1 = -log(c) = -log(0.1). However, I still don't know how to determine what percentage of the substance should be protonated.
The possible answers are
a) 99.9%
b) 90%
c) 33.3%
d) 0.1%
e) 10^-4 %
My first thought would be c because of the previously mentioned circumstances (weak acid, pH4), but it is really just a "clever" guess rather than proven
(1 rating, 1 votes, rated)You need to be a registered member to rate this.
Loading...
To answer the question, we can use the Henderson-Hasselbalch equation, which establishes a relationship between the pH value and the pKs value of an acid.
The Henderson-Hasselbalch equation is:
'pH = pKs + log10 ([A-]/[HA]]'
Here is:
pH of the solution,
– pKs the pKs value of the acid,
– [A-] the concentration of the base and
– [HA] the concentration of acid
In this case, the pKs value is 7 and the pH is 4. As we are interested in the protonated form, we are looking for [HA]. If we convert the equation to [HA], we get:
`[HA] = [A-] * 10^(pKa – pH)`
We assume that [A-] = 1 (ie, we consider the concentration of the base as a unit), then [HA] = 10^(7 – 4) = 10^3 = 1000.
This means that the protonated form of the substance is 1000 times more concentrated than the deprotonated form.
To calculate the percentage of protonated form, we use the following formula:
`%HA = ([HA] / ([HA] + [A-]) * 100`
If we insert [HA] = 1000 and [A-] = 1, we receive:
`%HA = (1000 / (1000 + 1)) * 100 = 99.9%`
Therefore, the substance is present in the protonated form at a pH of 4 to about 99.9%.
Henderson-Hasselbalch equation is now listed in the brain. Mathematics came into play. I understood the result and how to do it. I did not understand how to assume that the concentration is [A-] = 1.
The concentration can be used for 1.
Yes, because it doesn't matter how concentrated the substance is, the percentage of protonated and deprotonated form depends only on the pH (if it is given).
So just think so I have something I can count on?
In addition to the very correct answer from LeBonyt, I will show you a quick procedure:
The pH is three orders of magnitude (7-4=3) below the pKa value of the acid. So the answer has something to do with 10 ̄3=1⁄1000=0.1%. Since we are more acidic than the pKa value, the greater part of the acid must be protonated (it is precisely at the pKa value that acid and base form are present exactly in a ratio of 1:1). So we have the smaller part, ie 0.1%, in the base form (deprotonated) and the larger part (99.9%) in the acid form (protonated).