The pK_a of TRIS is HEPES is 7.55. What is the pH of a buffer prepared by mixing 200. mL of 0.20 M HEPES in its acid form with 100. mL of 0.20 M HEPES in it basic form? A) 6.55 B) 7.25 C) 7.85 D) 8.55

Question

Quizplus · Accepted Answer

First, we need to calculate the ratio of [HEPES] in acid form to [HEPES] in basic form in the buffer solution:

pH = pKa + log([base]/[acid])
7.55 = pKa + log([base]/[acid])
log([base]/[acid]) = 7.55 - pKa
log([base]/[acid]) = 7.55 - 7.5
log([base]/[acid]) = 0.05
[base]/[acid] = 10^0.05
[base]/[acid] = 1.13

This means that for every 1 molecule of HEPES in its acid form, we have 1.13 molecules of HEPES in its basic form.

Next, we need to calculate the concentration of HEPES in the buffer solution:

moles HEPES acid = 0.20 M x 0.200 L = 0.040 moles
moles HEPES base = 0.20 M x 0.100 L = 0.020 moles
total moles HEPES = 0.040 moles + 0.020 moles = 0.060 moles
total volume = 0.200 L + 0.100 L = 0.300 L
[HEPES] = 0.060 moles / 0.300 L = 0.20 M

Finally, we can use the Henderson-Hasselbalch equation to calculate the pH:

pH = pKa + log([base]/[acid])
pH = 7.55 + log(1.13)
pH = 7.55 + 0.05
pH = 7.60

Therefore, the answer is choice B (7.25).

The PKa of TRIS Is HEPES Is 7

The PK_a of TRIS Is HEPES Is 7