Question

You wish to prepare a buffer containing CH3COOH with a pH of 5.14. If the pka of acetic acid is 4.74, what ratio of

[CH3COO-]/[CH3COOH] must you use?
A 0.70
B 4.0
C 0.40
D 2.0
E 2.5

Answer 1

E

Step-by-step explanation:

Recall the Henderson-Hasselbalch equation:

`\displaystyle \text{pH} = \text{p}K_a + \log \frac{[\text{Base}]}{[\text{Acid}]}`

For a pH of 5.14 with the given pKa of 4.74, solve for the ratio between acetate and acetic acid:

`\displaystyle \begin{aligned} (5.14) & = (4.74) + \log\frac{[\text{CH$_3$COO$^-$}]}{[\text{CH$_3$COOH}]} \\ \\ \log\frac{[\text{CH$_3$COO$^-$}]}{[\text{CH$_3$COOH}]}& = 0.40 \\ \\ \frac{[\text{CH$_3$COO$^-$}]}{[\text{CH$_3$COOH}]} & = 10^(0.40) \\ \\ & = 2.5\end{aligned}`

In conclusion, the answer is E.