Question

Which weak acid would be best to use when preparing a buffer solution with a ph of 9.20?

Answer 1

Answer:

The acid with about $Ka=6.31x10^{-10}$.

Explanation:

Hello,

By assuming that the dissociation of a weak acid is:

$H^+X^-H^++X^-$

From the following equation one computes pKa as long as the concentration of the conjugate base equals the concentration of the acid for this buffered solution:

$pH=pKa+Log(\frac{[Conj.Base]}{[Acid]})\\pKa=pH-Log(1)\\pKa=pH=9.2\\Ka=10^{-9.2}\\Ka=6.31x10^{-10}$

So the acid will be that with a $Ka=6.31x10^{-10}$ or close since the possible answers are not shown.

Best regards.

Answer 2

Answer: -

6.31 x 10⁻¹⁰

Explanation: -

The equation for buffer pH

pH = pKa + log $\frac{Conjugate Base}{Acid}$

If [Conjugate base] = [Acid], then preparing a buffer is best.

Then pH = pKa

pKa = - log Ka

Ka = $10^{-9.2}$

= 6.31 x 10⁻¹⁰