Question

you select the amino acid Cysteine (pKa=8.2) and dissolve 0.1 moles in 1 liter of water in order to make a buffer solution. You then adjust the pH to 7.2, but then accidentally add 0.03 moles of HCl to the solution. is the solution still a good buffer?

Please help me understand how to use the number of moles added to solve the problem. I can't seem to understand how the amount of moles added relates to the amino acid's ability to be protonated or not. Thank you in advance!

The answer says: Yes-enough acid was added to reach the Carboxyl group buffering region where you have sufficient weak acid and conjugate base to resist a change in pH if strong acid or base were added.

However, how do you know that enough acid was added to reach the carboxyl buffering region?

Answer 1

First:

Amino acids --> NH2 + H+ --> NH3+

Carboxilic acids --> COO- + H+ --> COOH

therefore, addition of H+ does changes protonation of both groups

fr Cysteine, pKa = 8.2 fo the amino acid, therefore, we are focusing ou attention in the protonation:

NH2 + H+ --> NH3+

then.. after addition of:

HCl --> H+ ions are in solution

if

pH = pKa + log(cnjugate/acid)

when adding 0.03 mol --> 30 mmol

this does compares with the 0.1 --> 100 mmol of cysteine

therefore, expect buffer formation

those 30 mmol --> 30 mmol of NH3+ formed

100-30 = 70 mmol of cysteine left

so

NH2 and NH3+ are present, which eans they can still be used in th ebuffer