This question has multiple parts. Work all the parts to get the most points. Giv
ID: 540100 • Letter: T
Question
This question has multiple parts. Work all the parts to get the most points. Given a solution of 0.300 M HEPES in its fully protonated form, and ready access to 0.200 MHCL, 0.200 M NaOH and distilled water, complete the questions below to describe the preparation of 1 L of 0.0300 M HEPES buffer solution at pH 7.70 The structure of HEPES is shown below: O, o The sulfonic acid group has a p of around 3, and the ring nitrogen has a pKa of 7.55. i) Alter the structure of HEPES to show it in its fully protonated form. This form is monoprotonated.
Explanation / Answer
The sufonic acid group present has pka value 3 and ring nitrogen has pka value 7.55. HEPES in its fully protonated form would have its tertiary nitrogen fully protonated and get possitively charged and protonated uncharged sulfonic acid group is present. So to get this state, we must added HCl solution making the chloride salt of HEPES. And then to bring the pH 7.8 we must low it by adding base NaOH to deprotonate sulfonic acid group. And to deprotonate tertiary nitrogen we must add additional NaOH solution.
Now to make 0.03 M buffer solution we need 100 ml of 0.3M stock solution.
Now to calculate amount of base needed to adjust the pH 7.70 can be calculated by Handerson-Hasselbach equation.
pH = pka2 + log[HEPES1-]/ [HEPES-H]
7.70 = 7.55 + log[HEPES1-]/ [HEPES-H]
log[HEPES1-]/ [HEPES-H] = 7.70-7.55= 0.15
[HEPES1-]/ [HEPES-H] = 1.413
Ratio of [HEPES1-]/ [HEPES-H] = 1.413
As we know
[HEPES1-] + [HEPES-H] = 0.03
1.413 X [HEPES-H] + [HEPES-H] = 0.03
[HEPES-H] = 0.03 / 2.413
[HEPES-H] = 0.0124M
Conc of [HEPES-H] in final buffer solution is 0.0124M
[HEPES1-] + 0.0124 = 0.03
[HEPES1-] = 0.03 -0.0124
[HEPES1-] = 0.0176 M
Concentration of [HEPES1-] in final buffer solution is 0.0176M.
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