You are asked to prepare a buffer of pH 6.70 for a biological assay. Using the t

Question

You are asked to prepare a buffer of pH 6.70 for a biological assay. Using the table of pKa

You are asked to prepare a buffer of pH 6.70 for a biological assay. Using the table of pKa's in the experimental procedure (Part 4), select an appropriate acid/base pair and clearly state the pair that you selected. Calculate the volume of the acid solution and the volume of the conjugate base solution that would be needed to prepare a buffer with a total volume of 500. mL. Assume all solutions are 1.00 M. Show all of your work. Can someone please help me with this question? I have no idea what's going on. I'm pretty sure you have to use dihydrogen phosphate/hydrogen phosphate and I can figure out the ratio of base concentration to acid concentration as .309 but can't actually find the volume of acid and base needed.

Explanation / Answer

we will use pair 3 s its pKA is nearest to the required pH

dihydrogen phosphate (H PO -) 24

hydrogen phosphate (HPO 2-) 4

7.21

now

pH = pKa + log10([salt]/[acid])

6.7 = 7.21 + log10([salt]/[acid])

[salt]/[acid] = 0.309

as both have concentration = 1 M

and total volume (500ml) is same

the molar ratio salt/acid = 0.309

so if moles of acid = x

moles of salt = 0.309x

moles = molarity*volume

as molarity = 1

moles = volume

so 0.309x + x = 0.5................(500ml = 0.5 L)

x = 0.3819 L

so volume of acid = 381.9 ml.............................di hydogen phosphate

and volume of salt = 500-381.9 = 118.1 ml.............................hydrogen phosphate

dihydrogen phosphate (H PO -) 24

hydrogen phosphate (HPO 2-) 4

7.21

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