Question

a) What is H₂CO₃* ? Note we dropped the use of the * in doing water chemistry problems and use an artificially small Ka1 value (which might seem confusing unless you had a water chemistry course). And what aqueous species dominates in this definition?

b) Derive the relationship between total carbonate concentration and partial pressure of CO₂(g) in equilibrium with the water in an open system which is given as,

C_T,CO3 = K_H P_CO2/a_o

c) You have obtained a ground water sample at pH 6.3 with an alkalinity of 2.0 meq/L associated entirely with carbonate type species. You left it on a table until it equilibrated with normal air overnight and noted that the pH went up significantly while the total carbonate concentration decreased.   Why ? Show reactions/phenomena/sketch/draw cartoon. NO EQUATIONS BUT REACTIONS AND CHEMISTRY PLEASE.

d)   What is the alkalinity after the water in c) is equilibrated with air?

Answer 1

a) h2co3 is carbonic acid , its also called solution of carbon dioxide in water in water ( carbonated water ).. generally strong acids have smaller ka1 values.e.g h2so4, hcl , HNO3 etc .

b)In the presence of gaseous CO2, dissolved CO2 exchanges with CO2 gas:

CO2(g) + H2O ---->CO2(aq) + H2O

CO2(aq) + H2O ------>H2CO3

where g and aq refer to the gaseous and dissolved phase, respectively. Although the concentration of CO2(aq) far exceeds that of dissolved H2CO3 .we denote the concentration of all dissolved CO2 by [H2CO3]. The equilibrium condition between the phases is quantified by the molar solubility Ko (Henry's law)

K0 = [H2CO3]/PCO2

c) Here we are dealing with an open system to the atmosphere (exchange with atmospheric CO2) . PCO2 is constant. The system is closed to CaCO3, so that the carbonate alkalinity (=the concentration of positive metal ions) is constant.Starting with the known value of the atmospheric partial pressure of CO2, the dissolved CO2 concentration is obtained, and from that the pH dependent other concentrations

d)alkalinity of water found to be 50 mg/l