Question

Blood contains a buffer of carbonic acid (H2CO3) and hydrogen carbonate ion (HCO3-) that keeps the pH at a relatively stable 7.40. What is the ratio of [HCO3-] / [H2CO3] in blood? Ka1 = 4.30x10-7 for H2CO3.

Answer 1

Solution:

Given data --> pH = 7.40, K_a = 4.30 * 10^-7

The general representation of equilibrium constant is given by,

K_a = [H⁺][A^-] / [HA]

for blood the above expression deduces to,

Ka = [H⁺][HCO3^-] / [H2CO3] -------- (1)

on rearranging equation (1) for the ratio is given by,

[HCO₃^-] / [H₂CO₃] = Ka / [H⁺] ---------(2)

by substituting the given values in equation (2) we get,

{pH = -log [H⁺] = 7.40 so [H⁺] = 10^-7.4 = 4*10^-8 M }

[HCO₃^-] / [H₂CO₃] = (4.30 *10^-7) / (4*10^-8)

[HCO₃^-] / [H₂CO₃] = 10.75 = 11 (approx)