The leaf compost jar and the control jar both contained the same initial volume

Question

The leaf compost jar and the control jar both contained the same initial volume and concentration of NaOH, Explain why the control jar required volume of HCl than the leaf compost jar did to reach the end point of the titration. A pair of students forgets to precipitate the corbonate ions in their leaf compost jar's NaOH solution prior to tha HCl titration. How would this affect the perceived amount of NaOH present in the titrated? The perceived amount of NaOH would be higher that what it actually is. The perceived amount of NaOH would be lower that what it actually is. Explain your answer. How would forgetting to precipitate the carbonate the carbonate ions affect the perceived amount of CO_2 given off by the leaves? The amount of CO_2 would be perceived higher than what it actually is. The amount of CO_2 would be perceived lower than what it actually is. Explain your answer.

Explanation / Answer

You haven’t uploaded the complete experiment; hence I am answering based on the assumption that you did an experiment which released CO2 from the leaves by way of oxidation.

1) The CO2 released by aerobic oxidation of the leaves dissolves in water to form carbonic acid.

CO2 (g) <====> CO2 (aq)

CO2 (aq) + H2O (l) <====> H2CO3 (aq)

Carbonic acid is a weak diprotic acid and undergoes acid-base neutralization with the strong base, NaOH. Therefore, part of the NaOH is consumed to neutralize the generated carbonic acid and the remaining NaOH requires HCl for complete neutralization.

In the control jar, no CO2 and hence carbonic acid is generated and hence a greater volume of HCl is required to completely neutralize the NaOH.

2) (b) The perceived amount of NaOH would be lower than what it actually is.

The reason is that we are estimating the amount of NaOH by titrating the same with a HCl solution of known concentration. Due to the neutralization reaction of carbonic acid stated in part (1) above, some of the NaOH is consumed and HCl will have a lower amount of NaOH to react with. Therefore, a lower amount of NaOH will be recorded.

3) (a) The amount of CO2 would be perceived higher than what it actually is.

The reason is that the solution contains dissolved CO2 as carbonic acid. Since the students failed to precipitate all the carbonate ions, some portion of the carbonate dissolves in water forming carbonic acid which decomposes to CO2 and gives a higher value.

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