You have the following acids and their conjugate bases available:
Acetic Acid: CH 3COOH/CH3COO-; Ka = 1.8 × 10-5
Carbonic Acid: H2CO3/HCO3-; Ka = 4.2 × 10-7
Hydrofluoric acid: HF/F-; Ka = 7.2 × 10-4
Which of these acid/conjugate base pairs would be the best choice for a buffer in the range of pH 4.5?
a. acetic acid
b. carbonic acid
c. hydrofluoric acid
I believe B. Please help.
B is wrong. A is right. Typically you want to pick pKa withing +/- 1 of the desired pH.
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i think the answer is A because it's pKa is the closest to pH 4.5.
Christine! Are you doing AP Chem with k12?
Finally finished this course too
To determine which acid/conjugate base pair would be the best choice for a buffer in the range of pH 4.5, we need to consider the Henderson-Hasselbalch equation:
pH = pKa + log([A-]/[HA])
Where:
pH = the desired pH of the buffer
pKa = the logarithmic acid dissociation constant of the weak acid
[A-]/[HA] = the ratio of the concentrations of the conjugate base ([A-]) to the weak acid ([HA])
In this case, the desired pH is 4.5. We can rearrange the Henderson-Hasselbalch equation to solve for the ratio [A-]/[HA]:
[A-]/[HA] = 10^(pH - pKa)
Now let's calculate this ratio for each acid/conjugate base pair:
For acetic acid (CH3COOH/CH3COO-):
pKa = 1.8 × 10^-5, pH = 4.5
[A-]/[HA] = 10^(4.5 - (-5)) = 10^9.5
For carbonic acid (H2CO3/HCO3-):
pKa = 4.2 × 10^-7, pH = 4.5
[A-]/[HA] = 10^(4.5 - (-7)) = 10^11.5
For hydrofluoric acid (HF/F-):
pKa = 7.2 × 10^-4, pH = 4.5
[A-]/[HA] = 10^(4.5 - (-4)) = 10^8.5
Comparing the values of [A-]/[HA], we can see that the highest value is for carbonic acid (H2CO3/HCO3-) with a ratio of 10^11.5. Therefore, the best choice for a buffer in the range of pH 4.5 would be carbonic acid. The answer is b.