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Homework Help: Bromothymol Blue Equilibrium

  1. Nov 23, 2015 #1
    1. The problem statement, all variables and given/known data
    Bromothymol blue is an acid-base indicator. When in solution, it forms the following equilibrium:
    HInd(aq) ⇌ H+(aq) + Ind-(aq)
    What side of the equilibrium is favored while the solution is in the bromothymol blue bottle?

    2. Relevant equations

    Keq = [(C)c(D)d]/[(A)a(B)b]

    3. The attempt at a solution

    I think the equilibrium favors the reactants because since it is blue, then bromothymol blue is a base so there is small amounts of H+ ions so the K expressions is a small number divided by a bigger number which results in a small number.

    Can anyone confirm that my thinking is correct? Thanks!
  2. jcsd
  3. Nov 24, 2015 #2


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    Staff: Mentor

    Yes. Bromothymol blue is a weak acid, with pKa around 7.
  4. Nov 25, 2015 #3


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    Homework Helper
    Gold Member

    The equation you give is not the relevant one. The right one would also suggest answer.
  5. Nov 25, 2015 #4
    Thank you, but why is Bromothymol Blue an acid even though it is blue?
  6. Nov 26, 2015 #5


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    Staff: Mentor

    You are mistaking several things at the same time. Being an acid doesn't mean its solutions have low pH. First, it is a weak acid, second, its solutions - as used in the analytical chemistry - have very low concentration.

    What is important is that - as a weak acid - it dissociates, it has a conjugate base (which has a different color than the acid itself), and the ratio of concentrations of both forms depends on the solution pH.
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