Biochem/ pKa/ buffer question?

2020-08-12 1:03 am

Given a weak acid with a pKa of 5.5: 1) In what pH range would this acid be an effective buffer? 2) In a 0.18 M solution at pH 8.0, what fraction would be in the conjugate base form? 3) At pH 8.5, what fraction would be in the conjugate base form?

Thank you !

回答 (1)

hcbiochem

2020-08-12 1:14 am

1) A weak acid is an effective buffer with in 1 pH unit below to 1 pH unit above its pKa. So, for this acid, the effective range would be 4.5 - 6.5.

2) Is this related to the weak acid with a pKa = 5.5?? Or did you mis-enter the original pKa?

pH = pKa + log [base]/[acid]
8.0 = 5.5 + log [base]/[acid]
[base]/[acid] = 316

So, for every molecule in the acidic form, 316 would be in the conjugate base form. As a proportion in the basic form, 316/317 or 99.68% in the basic form

3) 8.5 = 5.5 + log [base]/[acid]
[base]/[acid] = 1000
1000/1001 = 0.9990 or 99.90% in the basic form

👍 0 👎 0