Question

You are to make 100.0ml of a 0.50 m acetate buffer (pka = 4.72) at ph 5.00 using 0.50 m stocks of acetic acid and sodium acetate. 6. using henderson-hasselbalch and the information given, calculate the value of [a-]/[ha].

Answer 1

$[\text{A}^{-}] / [\text{HA}] \approx 0.525$

Explanation:

The Henderson-Hasselbalch equation estimates the value of $[\text{A}^{-}] / [\text{HA}]$ of a buffer given its $pH$ and the acid dissociation constant of weak acid $\text{HA}$:

$pH \approx pK_{a} + \text{log}_{10} ([\text{A}^{-}] / [\text{HA}])$

Rearranging:

$[\text{A}^{-}] / [\text{HA}] \approx 10^{-pH} / 10^{-pK_a}$

The question states that

• $pH = 5.00$ and
• $pKa = 4.72$

Thus

$[\text{A}^{-}] / [\text{HA}] \approx 10^{-5.00} / 10^{-4.72} = 0.525$

Reference:

Gurinder Khaira and Alexander Kot (UCD), "Henderson-Hasselbalch Approximation," Chemistry Libretext