A) What is the aqueous phase CO2 concentration (mg/L) of a watersample that is not in equilibrium with the atmosphere and has a pH= 7.1, an alkalinity of 300 mg/L and a temperature of 25 degrees C?
This I was able to figure out, [CO2(aq)] = 8.74 x 10^ -4 M
Use pH to find [H+].
Alk = [HCO3 (-)] at this pH so I converted 300 mg/L into M.
Using the equilibrium expression and the Ka of it given in class(Ka = 4.47 x 10^ -7 M), I found [H2CO3 *] = 8.74 x 10^ -4 M.
In natural waters, [H2CO3 *] = [CO2(aq)] so therefore [CO2(aq)] =8.74 x 10^ -4 M.
note: [H2CO3 *] just means CO2 (aq) and H2CO3 combined becausethey're difficult to distinguish quantitatively.
B) What would be the concentration (in atm) of CO2 in the gas phase of this system?