91Ó°ÊÓ

The pH of the buffer determined by the concentration ratio of $\left[A^{-}\right]$and localid="1654835921373" $\mathrm{\left[}\mathrm{HA}\mathrm{\right]}$ by the following Henderson-Hasselbalch equation: $\mathrm{pH}={\mathrm{pK}}_{\mathrm{a}}+\log \left(\left[{\mathrm{A}}^{-}\right]/\left[\mathrm{HA}\right]\right)$.

a)

Because HEPES ($N-2$Hydroxyethylpiperzine- $N^{\text{'}}-2$-ethanesulfonic acid) is a weak acid, its pH is less than 7 .

If we dissolve this acid in water, the solution will be acidic, requiring the addition of $NaOH$- a buffer is generated, and the pH will be raised to 7.45 .

b)

To make a HEPES buffer with $pH=7.45$, follow these steps:

The following information is provided:

So,

$$\begin{gathered} molarityofHEPES=0.05MvolumeofHEPES \\ =0.250L \\ molesofHEPES=molarity×volume \\ =0.0500M×0.250L \\ =0.0125moles \end{gathered}$$

After that, combine $0.0125$moles of HEPES with $250mL$of water, measure pH, and add $NaOH$ until $pH=7.45$.