Chemistry CO2 and H2CO3 Concentrations in Aqueous Solution: Understanding the Relationship

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The discussion centers on the relationship between CO2 and H2CO3 concentrations in aqueous solutions. It is noted that while CO2 does not completely convert to H2CO3 upon dissolution, an equilibrium exists between CO2, H2O, and H2CO3. The rapid nature of this reaction allows for the assumption that all CO2 can be treated as converted to H2CO3 for calculation purposes. The presence of other ions in an alkaline solution can influence the behavior of carbonic acid. Ultimately, this assumption simplifies the problem-solving process without significantly affecting the accuracy of results.
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Homework Statement
A chemist dissolves 0.135 mole of CO2(g) in 2.50 L of 0.105 M Na2CO3. Calculate the pH of the resulting solution.
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pH
The solution says that the concentration of CO2 is equal to the concentration of H2CO3, which can be used to solve the rest of the problem. Does CO2 turn completely into H2CO3 when dissolved in aqueous solution? Thanks.
 
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Well the solution is a aqueous - it is also alkaline. So what is there in solution (what ions?) already when the CO2 is thrown in, and what is any carbonic acid molecule going to do in that solution? (What do acids do?)
 
i_love_science said:
The solution says that the concentration of CO2 is equal to the concentration of H2CO3, which can be used to solve the rest of the problem. Does CO2 turn completely into H2CO3 when dissolved in aqueous solution?

Actually we don't know - but it doesn't matter. There is definitely some equilibrium between CO2, H2O and H2CO3, but the reaction is so fast we can safely assume that equilibrium to be just part of the H2CO3 dissociation - it changes Ka a bit, but the value of Ka we OBSERVE already takes into account. So you can safely assume all carbon dioxide gets converted into carbonic acid and use this assumption as a starting point of the calculations, even if it is not entirely correct, the result will be OK.
 
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