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.
I don't see a question here. I assume you want directions for preparing the buffer. Let's say you want 1 L of the buffer.
pH = pKa + log (base)/(acid)
7.40 = pK2 + log (base)/(0.0100)
You will need to look up pK2 for H3PO4. solve the equation for (base).
Knowing you want 1 L of solution, you want mols = M x L = ?
Now you know mols NaH2PO4(the acid) and mols NaHPO4(the base). Calculate grams from g = mols x molar mass.
Weigh the amounts of acid and base and dissolve enough water to make 1 L.
I'm sorry the full question was:
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?
To prepare a buffer solution with a pH of 7.40 using a sodium phosphate buffer, we need to determine the appropriate concentrations of both the acid (H2PO−4) and the conjugate base (HPO2−4).
1. First, let's understand the Henderson-Hasselbalch equation, which is commonly used to determine the pH of a buffer solution:
pH = pKa + log([A−] / [HA])
Where:
- pH is the desired pH of the buffer solution.
- pKa is the negative logarithm of the acid dissociation constant (Ka).
- [A−] is the concentration of the conjugate base.
- [HA] is the concentration of the acid.
In this case, since we want to determine the concentration of the conjugate base ([A−]), we rearrange the equation as follows:
[A−] = 10^(pH - pKa) * [HA]
2. Now, we know the desired pH (7.40) and the concentration of the acid ([HA]) as 0.0100 M. The pKa value for the sodium phosphate buffer can be found in reference books or online sources. For example, the pKa value for sodium phosphate is commonly around 7.21.
[A−] = 10^(7.40 - 7.21) * 0.0100
[A−] ≈ 0.020 M
Therefore, the concentration of the conjugate base (HPO2−4) in the sodium phosphate buffer should be approximately 0.020 M in order to achieve a pH of 7.40.
3. So, to prepare the buffer solution, you would mix a solution containing 0.0100 M of the acid (H2PO−4) and a solution containing 0.020 M of the conjugate base (HPO2−4) in the desired proportions.
Keep in mind that these calculations involve approximations and assumptions, and the actual pH might vary slightly due to factors such as temperature and other influences. It is recommended to double-check the pKa value and consult specific references for the sodium phosphate buffer system to ensure precise preparation.