The pH of human blood needs to be between 7.35 and 7.45. You want to prepare a buffer
solution that gives a pH of 7.40. You decide to use a sodium phosphate buffer: the acid is
H2PO−
4 and the conjugate base is HPO2−
4
. You want the concentration of the acid to be
0.0100 M.
1. If the initial H2PO−
4
concentration is 0.0100 M, what is the initial concentration of
HPO2−
4
that will give a pH of 7.40?
2. What is the maximum molarity of acid that this buffer can neutralize without the pH
dropping below 7.35?
3. What is the maximum molarity of base that this buffer can neutralize without the pH
going above 7.45?
Didn't I do parts of this a day or so ago for you. I know i did art 1.
What is the trouble you're having.
To answer these questions, we can make use of the Henderson-Hasselbalch equation, which relates the pH of a buffer solution to the pKa of the acid and the ratio of the concentrations of the acid and its conjugate base. The Henderson-Hasselbalch equation is as follows:
pH = pKa + log([base]/[acid])
1. To calculate the initial concentration of HPO2−
4
that will give a pH of 7.40, we first need to determine the pKa of the acid H2PO−
4
. The pKa can be found in a reference table or obtained experimentally. In this case, let's assume the pKa of H2PO−
4
is 7.21.
Using the Henderson-Hasselbalch equation, we can rearrange it to solve for [base]/[acid]:
pH - pKa = log([base]/[acid])
7.40 - 7.21 = log([base]/0.0100)
0.19 = log([base]/0.0100)
To find the ratio [base]/0.0100 M, we need to use the logarithm calculator:
0.19 = log([base]/0.0100) => 10^0.19 = [base]/0.0100 => [base] = 0.0100 * 10^0.19 = 0.0149 M
Therefore, the initial concentration of HPO2−
4
that will give a pH of 7.40 is approximately 0.0149 M.
2. To determine the maximum molarity of acid that this buffer can neutralize without the pH dropping below 7.35, we need to consider the acid-base equilibrium reaction between H2PO−
4
and HPO2−
4
.
H2PO−
4
⇌ HPO2−
4
+ H+
In this equilibrium, the pH of the buffer lies between the pKa values of the acid and its conjugate base, which means that the concentration of the acid and its conjugate base are roughly equal.
Given that the initial concentration of the acid is 0.0100 M, the maximum molarity of acid that this buffer can neutralize without the pH dropping below 7.35 is approximately 0.0100 M.
3. Similarly, to determine the maximum molarity of base that this buffer can neutralize without the pH going above 7.45, we consider the same acid-base equilibrium reaction.
Using the same reasoning as in question 2, the concentration of the conjugate base and acid are roughly equal. Therefore, the maximum molarity of base that this buffer can neutralize without the pH going above 7.45 is approximately 0.0100 M.