Question

1. Published findings in an article written by G. Schmitz report that measured pH values for 0.10 M acetic acid and for acetic acid/acetate buffer solutions agree well with the calculated values. (For our purposes we will say that good agreement means that the measured and the calculated values agree with each other to within about three tenths of a pH unit or less.) However, measured pH values for 0.10 M sodium acetate are found to be always significantly lower (approximately 1 pH unit too low) than the calculated value. Schmitz proposes that the measured pH of 0.10 M sodium acetate solutions is too low due to the dissolution of CO2 in the solution. In his article he makes the following statement: “In aqueous solutions in equilibrium with air containing 350 ppm of CO2, the dissolved CO2 concentration is approximately 10—5 M.” a) Explain why, in two to three sentences, Schmitz’s proposal that dissolved CO2 is the cause of the low measured pH of 0.10 M sodium acetate is reasonable. (Hint: Think about what is formed in solution when CO2 reacts with water. You may find it useful to use your textbook as a reference!)

b) What is formed when CO2 reacts with water? You may find it useful to use your textbook as a reference.

i) Give a chemically reasonable explanation as to why dissolved CO2 does not cause the measured pH of the
acetic acid/acetate buffer solution to deviate significantly from the calculated value.

ii) Give a chemically reasonable explanation as to why dissolved CO2 does not cause the measured pH of the
0.10 M acetic acid to deviate significantly from the calculated value.

iii) Explain why Schmitz’s proposal that dissolved CO2 is the cause of the low measured pH of 0.10 M sodium
acetate is reasonable. (Hint: Think about what is formed in solution when CO2 reacts with water and the
various reactions (equilibria) involved. This question is complex, and requires some thoughtful analysis.
Discuss the question with fellow classmates, your instructor, etc....)

Answer 1

ANS (a). When CO₂ reacts with H₂O in water , carbonic acid is formed. Carbonic acid is a weak acid which releases proton in the solution which lowers the pH. If CO₂ is not considered, H₂O will be donating protons but will be generating OH^- in the solution which will increase the pH.

(b) . Carbonic acid is formed according to the following reaction.

CO₂ + H₂O H₂CO₃

(i) In presence of buffer the dissociation of H₂CO₃ is inhibited because of presence of common ion.Since acid is already present in the solution so dissociation of carbonic acid giving a proton is reduced and hence there is not significant effect on pH of the solution.

(ii) Because acetic acid itself is a weak acid which dissociates in solution giving off a proton and carbonic acid is also a weak acid. Already presence of H+ in solution stops another acid to dissociate and so concentration of H+ doesnt vary much which results in insignificant change in pH of solution.

(iii) CO₂ reacts very rapidly with H₂O because of former having high degree of unsaturation and the latter being a good nucleophile which carries out the formation of carbonic acid.Formation of any new specie in the solution will bring about changes in the equilibrium and thus affect the pH. Since we are using a basic solution so H+ will be neutralised by the basic group and so dissociation of carbonic acid will increase further. So from a theoretical as well as practical point of view, this explanation is reasonable.