Answer to Question #42665 in Chemistry for Austeja

Question

In 10 mL of blood sample pH was 7,56. A high amount of acid was added into the blood. After that volume of CO2 in a sample was 6,13mL. pKa is 6,1. Find concentrations of CO2 and HCO3- in blood.

Please, help me, I almost understand how everything should be calculated, but somewhere I keep thinking faulty I guess because can't get the possible answers anyhow...

Answer 1

Find the amount of substance of CO₂. Assume that the volume was measured at ambient conditions (298.15 K, 100 kPa).
n(CO₂) = pV/RT = 2.473*10^-4 mol
Assume that the only substance responsible for blood pH is CO₂. Carbon dioxide dissociates in water according to the following equation:
CO₂ + H₂O --> H⁺ + HCO₃^-
We have a buffer system, and its pH can be calculated from the following equation:
pH = pK_a + lg [A^-]/[HA]
where [A^-] is the concentration of bicarbonate ion HCO₃^-, and [HA] is the concentration of free CO₂.
[A^-]/[HA] = 10^(pH - pK_a) = 28.84
The total amount of CO₂ and HCO₃^- in the solution is equal to the amount of evolved CO₂. Therefore we can find the total concentration of these particles:
c(CO₂ + HCO₃^-) = n(CO₂)/V(solution) = 2.473*10^-4/0.01 = 0.02473 mol/L
Let x denote the concentration of free CO₂ in the solution. Hence the concentration of HCO₃^- ions is 28.84x
x + 28.84x = 0.02473
x = 8.29*10^-4 mol/L
28.84x = 2.39*10^-2 mol/L

Answer:
c(CO₂) = 8.29*10^-4 mol/L
c(HCO₃^-) = 2.39*10^-2 mol/L