1. Published findings in an article written by G. Schmitz report that measured p
ID: 1009875 • Letter: 1
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) Give a chemically reasonable explanation, in two to three sentences, 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.
c) Give a chemically reasonable explanation, in two to three sentences, 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.
d) Compare your pH data with your calculated pH values for the three solutions. Are your experimental results consistent with the findings reported by Schmitz?
Explanation / Answer
When CO2 dissolves in water it forms carbonic acid. H2CO3. It can dissociates as H+ and CO3 ^2-
These H+ can lower the pH value in the medium.
But this doesnt cause the measured pH of the buffer solution. Because a buffer solution is a solution which contains a weak acid and its conjugated weak base in which the whole complex can act as a barrier to pH change with the addition of a small amount of acid or base. When carbonic acid increases acetate ions can take H+ of carbonic acid and neutralize them. Then the dissolved CO2 doesn't affect the pH of buffer solution.
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