An aqueous solution containing 0.2M of sodium hydroxide is allowed to equilibrat

Question

An aqueous solution containing 0.2M of sodium hydroxide is allowed to equilibrate with the atmosphere in an analytical laboratory where the temperature is kept at 25 degree C. The partial pressure of carbon dioxide in the atmosphere, P_CO2, is 10^-3.5 atm, and the Henry's Law constant for carbon dioxide K_H = 10^-1.5 mol.L/atm. Determine the pH and the total acid concentration, C_T, CO3, of the solution when equilibrium with the atmosphere is reached. How do you expect the concentration C_T, CO3 to vary with initial concentration of NaOH in solution?

Explanation / Answer

[H2CO3] = Kh.PCO2

[H2CO3] = 9.99 X 10-6 mol.L

K1 =  [H+] [HCO3-] / [H2CO3]

K1.[H2CO3] = [H+]2

[H+]2 = 5.01 X 10-12

[H+] = 2.237 X 10-6 and [HCO3-] = 2.237 X 10-6

pH = 5.65

K2 = [CO32-] [H+] / [HCO3-]

[CO32-] = 4.69 X 10-11

Total acid concentration = [CO32-] + [HCO3-] +  [H2CO3]

Total acid concentration = 1.223 X 10-5

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