In the bloodstream, the effective pKa for carbonic acid is 6.35. If the pH of the blood is 7.40, what percentage of the blood buffer is present as bicarbonate

The 91.8% is present as bicarbonate ion

Explanation:

It is possible to find pH of a buffer using H-H equation:

pH = pKa + log [A⁻] / [HA]

Where pH is 7.40; pKa is 6.36, [A⁻] is concentration of bicarbonate ion and [HA] concentration of carbonic acid

7.40 = 6.35 + log [A⁻] / [HA]

1.05 = log [A⁻] / [HA]

11.22 = [A⁻] / [HA] (1)

As the question is in therms of percentage:

[A⁻] + [HA] = 100 (2)

Replacing (2) in (1):

11.22 = [A⁻] / 100 - [A⁻]

1122 - 11.22[A⁻] = [A⁻]

1122 = 12.22[A⁻]

91,8% = [A⁻]