Question

Would you need$\mathit{N}\mathit{a}\mathit{O}\mathit{H}\mathbf{}\mathit{o}\mathit{r}\mathbf{}\mathit{H}\mathit{C}\mathit{I}\mathbf{}$to bring the pH of $\mathbf{0}\mathbf{.}\mathbf{0500}\mathit{M}$HEPES (Table 9-2) to 7.45?

(b) Describe how to prepare $\mathbf{0}\mathbf{.}\mathbf{250}\mathit{L}\mathbf{}\mathit{o}\mathit{f}\mathbf{}\mathbf{0}\mathbf{.}\mathbf{0500}\mathit{M}\mathbf{}$HEPES, $\mathit{p}\mathit{H}\mathbf{7}\mathbf{.}\mathbf{45}$.

Short answer

a) $NaOH$is added to bring pH to 7.45 .

b) Combine HEPES and water and add $NaOH$until it become $pH=7.45$.

Step-by-step solution

Concept used.

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: $\mathrm{pH}={\mathrm{pK}}_{\mathrm{a}}+\log \left(\left[{\mathrm{A}}^{-}\right]/\left[\mathrm{HA}\right]\right)$.

Either  NaOH or HCI   is needed to bring the pH of 0.0500M HEPES to 7.45.

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 .

The preparation of  0.250L of 0.0500M  HEPES to pH7.45.

b)

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

The following information is provided:

So,

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

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