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Homework Help: Acid/Base Solution question

  1. Apr 14, 2010 #1
    1. The problem statement, all variables and given/known data

    The question was:

    When you add solid NaHCO3 to water the solution becomes basic. As you stir the mixture to dissolve all the NaHCO3 (or as the solution sits) bubbles of CO2 are released. This occures because HCO3- ions react with one another to a small extent according to:

    2HCO3- ---> CO3^2- + H2CO3
    H2CO3 <----> H20 + CO2

    The conjugate acid/base of HCO3- are H2CO3 and CO3^2- respectively. When CO2 is lost from the solution, the concentration of _____ (acid/base) decreases. Therefore, the more CO2 that is lost, the ____(more/less) basic the solution becomes.

    2. Relevant equations

    3. The attempt at a solution

    Looking at the second equation, I want to say that if CO2 decreases, this would drive the reaction to the right to compensate for the loss, decreasing H2CO3, the acid. Therfore, the concentration of acid decreases and the solution becomes more basic. However, other students in my class for the most part disagreed with me, and I can't seem to find out what is the right answer. If I am right, can someone let me know? And if I am wrong, can someone explain to me why??

    Thank you!
  2. jcsd
  3. Apr 16, 2010 #2


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    Staff: Mentor

    You are right - when CO2 escapes the solution it is technically equivalent to removing part of the acid. pH must go up.

    Change in amount of acid always means change in pH, and this change always go in the logical direction - more acid, lower pH, less acid, higher pH. When you start with NaOH solution exposed to the air quite the opposite happens - such solution absorbs CO2 from the air. That means adding acid, so pH goes down.

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