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Answer to Question #42665 in Other Chemistry for Austeja

Question #42665
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...
Expert's answer
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 = pKa + lg [A-]/[HA]
where [A-] is the concentration of bicarbonate ion HCO3-, and [HA] is the concentration of free CO2.
[A-]/[HA] = 10^(pH - pKa) = 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

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

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