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Homework Help: Equilibrium and Highest [A]

  1. Apr 26, 2008 #1
    [SOLVED] Equilibrium and Highest [A]

    Relevant equation
    Consider the following equilibria in 0.10 moldm−3 carbonic acid.
    (In attachment)
    Which species is present in the highest concentration?
    A. H2CO3 (aq)
    B. H+(aq)
    C. H(CO3)− (aq)
    D. (CO3)2− (aq)
    The attempt at a solution
    The right answer is A but I would think that B would be the right answer because as carbonic acid includes 2 H+ cations so doubling their concentration. Please, say your comment on this problem, why only A is right.

    Thank you in advance!

    Attached Files:

  2. jcsd
  3. Apr 26, 2008 #2


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    Homework Helper
    Education Advisor
    Gold Member

    Understand that carbonic acid is a weak acid, Ka = about 1.8*10^(-5). Based on this, what percent of the carbonic acid will disassociate?
  4. Apr 27, 2008 #3
    Good point. I got for [H+]= 10^-3, as this is much smaller than 0.1 so H2CO3 is in greatest concentration.

    Is it always with weak acids that the concentration of initial acid is highest?
  5. Apr 27, 2008 #4
    weak acids tend to remain as molecules in solutions, they ionise to a small extent.
  6. Apr 27, 2008 #5


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    Staff: Mentor

    Depends on their strength and concentration, there is no one, general rule. However, for obvious reasons (stoichiometry) once undissociated form fails below concentration of dissociated form, it is H+ that starts to dominate the solution :)

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