The concept of pH

Question 1

Henderson-Hasselbalch equation

Answer 1

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

Question 2

What happens to an acid when pKa = pH?

Answer 2

(Henderson-Hasselbalch) when [A-] = [HA], the acid is 50% dissociated and the equation simplifies to pH = pKa (because log 1 = 0) (the maximum buffer range runs at values close to pKa, from one pH unit below and above the pKa.

Question 3

buffer

Answer 3

A buffer is a weak/acid conjugate base pair that acts to minimize changes in pH after the addition of an acid or a base, by releasing or absorbing in accordance with the pKa (maximumum buffering range runs at values close to pKa, from one pH unit above and below it.

Question 4

What does having a low pKa mean?

Answer 4

Low pKa means H+ is easily dissociable. The pKa of most carboxyl groups is < 7.0, so they are termed acidic.
When pKa < pH then [HA] < [A-] ie A- (dissociated) is the predominant form in the solution.

Question 5

What does having a high pKa mean?

Answer 5

Low pKa means H+ is more tightly bound. The pKa of most amino groups is > 7.0, so they are termed basic. When pKa > pH then [HA] > [A-], ie. HA (associated/protonated) is the predominant form in the solution.

Question 6

When pKa > pH

Answer 6

When pKa < pH then [HA] < [A-] ie A- (dissociated) is the predominant form in the solution.
At pH 1 unite above pKa there is 91 %A- (deprotonated) and 9% HA.

Question 7

When pKa < pH

Answer 7

When pKa > pH then [HA] > [A-], ie. HA (associated/protonated) is the predominant form in the solution.
At pH 1 unit above pKa there is 91% (associated) and 9% A- (dissociated)