You need to use KH2PO4 and K2HPO4 to prepare a buffer with a pH of 7.45. Which of the following ratios of [base]/[acid] is required? For phosphoric acid, (H3PO4), Ka2 = 6.2 × 10–8.
Do I use the Hasselbach equation?
Yes.
7.45 = pKa2 + log (b/a)
b/a = ?
Yes, you can use the Henderson-Hasselbalch equation to determine the ratio of base to acid required for preparing a buffer with a specific pH.
The Henderson-Hasselbalch equation is:
pH = pKa + log([A-]/[HA])
Where pH is the desired pH of the buffer, pKa is the negative logarithm of the acid dissociation constant (Ka) of the acid involved, [A-] is the concentration of the conjugate base, and [HA] is the concentration of the acid.
In this case, you want to prepare a buffer with a pH of 7.45 using the acid H3PO4 (phosphoric acid) with Ka2 = 6.2 × 10–8.
To find the pKa of the acid, you need to take the negative logarithm (base 10) of the acid dissociation constant:
pKa = -log10(6.2 × 10–8)
pKa ≈ 7.21
Now that you have the pKa value, you can substitute it into the Henderson-Hasselbalch equation:
7.45 = 7.21 + log([A-]/[HA])
Rearranging the equation, you get:
log([A-]/[HA]) = 7.45 - 7.21
log([A-]/[HA]) ≈ 0.24
Now, to find the ratio of [base]/[acid], you need to take the antilog (base 10) of both sides of the equation:
[A-]/[HA] ≈ 10^0.24
[A-]/[HA] ≈ 1.58
Therefore, the required ratio of [base]/[acid] for the buffer would be approximately 1.58:1.
Yes, you can use the Henderson-Hasselbalch equation to determine the required ratio of [base]/[acid] for preparing the buffer with a target pH of 7.45. The Henderson-Hasselbalch equation is given by:
pH = pKa + log([base]/[acid])
In this case, the acid refers to H2PO4^- (formed by the dissociation of KH2PO4) and the base refers to HPO4^2- (formed by the dissociation of K2HPO4).
To use the Henderson-Hasselbalch equation, you need the pKa value of the acid. Given that the acid is phosphoric acid (H3PO4) and Ka2 = 6.2 × 10^-8, you can calculate pKa as follows:
pKa = -log(Ka2)
pKa = -log(6.2 × 10^-8)
Now that you have the pKa value, you can use the Henderson-Hasselbalch equation to solve for the ratio of [base]/[acid]:
7.45 = pKa + log([base]/[acid])
Substitute the pKa value you calculated into the equation and solve for [base]/[acid]:
7.45 = (-log(6.2 × 10^-8)) + log([base]/[acid])
Solving this equation will give you the required ratio of [base]/[acid] for your buffer solution.