The pH of earth’s water is con
The pH of earth’s water is controlled by the combination ofseveral equilibria found in water, as well as between water and theatmosphere. This problem considers a model for oceanic water. H2CO3+ CO3 yields 2HCO3 1) Calculate the equilibrium constant for thisequilibrium using Ka1 and Ka2 of carbonic acid. 3) Starting from apure solution of HCO3, how are the concentrations of H2CO3 and CO3related to each other? 4) Ocean water has an average pH of approx.8.1. How are the concentrations of H2CO3 and CO3 related to eachother in ocean water? 4) Do you think modeling ocean water as apure dilute bicarbonate ion solution a good model
Answer:
1) the equilibrium will be between
H2CO3 + CO3-2 –>2HCO3–
Equilibrium constant = K = [HCO3–]^2 /[H2CO3][CO3-2]
The Ka1 and Ka2 for H2CO3 will be
H2CO3 –> H+ + HCO3-
Ka1 = [H+] [ HCO3-] /[H2CO3]
HCO3- –> H+ + CO3-2
Ka2 = [CO3-2][H+] / [HCO3–]
so K = Ka1 / Ka2
2) Question missing
3) HCO3- + H+ –> H2CO3 (1/Ka1)
1/Ka1 = [H2CO3] / [HCO3-] [H+]
Ka1 = [HCO3-] [H+] / [[H2CO3]
pKa1 = pHCO3- + pH – pH2CO3
pHCO3- = pKa1 – pH + pH2CO3……(1)
HCO3- —> H+ +CO3-2 (Ka2)
Ka2 = [H+]'[CO3-2] / [HCO3-]
pKa2 = pH’ + pCO3-2 – pHCO3-
pHCO3- = pH’ + pCO3-2 -pKa2 … …(2)
So we can equate (1) and (2)
pKa1 – pH + pH2CO3 = pH’ +pCO3-2 -pKa2
So (CO3-2) = {(2Ka)/[H+]} X[HCO3–]
4) Average pH = 8.1
H2CO3 –> H+ + HCO3-
HCO3- –> H+ + CO3-2
[H+] = -antilog 8.1 = 7.94 X 10^-9
So if we know the concentratio of any species we can calcualteconcentration of CO3-2 accrodingly
4) yes the modelling water is a good model
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