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If you dissolved had a 0.5M Tris buffer (Molecular weight is 121.1grams/mole) at pH 6.5 and...

If you dissolved had a 0.5M Tris buffer (Molecular weight is 121.1grams/mole) at pH 6.5 and needed to adjust this solution to pH 8.0 how many would you get this solution to a pH of 8.0? Indicate what you would need to add, HCl or NaOH and what volume of a 1M solution of HCl or NaOH you would need to add to obtain a final pH of 8.0? Assume here you will end with a 1L solution.

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Answer #1

A Tris buffer at pH 6.5 would be converted to pH 8.0 by addition of base.

Using Hendersen-Hasselbalck equation,

pH = pKa + log([base]/[acid])

6.5 = 8.1 + log([base]/[acid])

[base] = 0.025[acid]

[acid] + [base] = 0.5 M

[acid] + 0.025[acid] = 0.5

[acid] = 0.49 M

[base] = 0.01 M

With, pH = 8.0

8.0 = 8.1 + log([base]/[acid])

[base] = 0.79[acid]

[acid] + [base] = 0.5 M

[acid] + 0.79[acid] = 0.5

[acid] = 0.28 M

[base] = 0.22 M

With volume of buffer solution = 1 L

M = mols

So, we have to bring [base] concentration from 0.01 to 0.22 mols

Volume of 1M NaOH to be added = (0.22 - 0.01)mol/1 M = 0.21 L

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