CO2 and H2CO3 Concentrations in Aqueous Solution: Understanding the Relationship

[i_love_science]

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.

Relevant Equations

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.

 

[epenguin]

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?)

 

[Borek]

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.