Chemistry-Help Dr. Bob222

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If a weak acid has a dissociation constant of 6.4 x 10^-6, what is the pH of a solution of this acid in a 0.10 molar concentration?

I still don't understand the equation you gave me. My textbook doesn't break it down like this, so I'm confused as to where to plug the 0.10 M in.

  • Chemistry-Help Dr. Bob222 -

    I don't remember how I worked this before. Hopefully what follows will duplicate what I did before. If you have trouble following, post the way your book does it and I'll try to explain it that way.
    We'll call the weak acid HA. Then
    .........HA ==> H^+ + A^-
    initial..0.10M...0......0
    change....-x....+x......+x
    final....0.10-x...x......x

    Ka = (H^+)(A^-)/(HA)
    6.4E-6 = (x)(x)/(0.10-x)
    Solve for x and convert to pH.

  • Chemistry-Help Dr. Bob222 -

    Thanks, totally still confused. I don't know how to solve for (x) since I don't know what the acid is?

  • Chemistry-Help Dr. Bob222 -

    The equation I'm trying to use looks like this:
    1.0 x 10^-14 divided by 6.4 x 10^-6 which gives me a solution of 1.5625 x 10^-9 which can't be right!

  • Chemistry-Help Dr. Bob222 -

    We'll call the weak acid HA. Then
    .........HA ==> H^+ + A^-
    initial..0.10M...0......0
    change....-x....+x......+x
    final....0.10-x...x......x

    Ka = (H^+)(A^-)/(HA)
    6.4E-6 = (x)(x)/(0.10-x)
    Solve for x and convert to pH.

    You don't need to know the name of HA, everything is here.
    6.4E-6 = (x)(x)/(0.1-x)
    6.4E-6 = x^2/(0.1-x)
    Often we can avoid a quadratic equation by making the simplifying assumption that 0.1-x = 0.1. Then
    6.4E-6*0.1 = x^2 and
    x = 8.0E-4 = (H^+)
    pH = 3.096 which rounds to 3.10.

    The inclusion of Kw in you response makes me wonder if the problem is the 0.1M SALT of a weak acid since the Kb of the salt IS Kw/Ka = 1x10^-4/6.4E-6. If your original post, however, is as above in the first post, pH = 3.1 is correct for this problem. I've gone back and re-read the post and it says nothing about the salt.

  • Chemistry-Help Dr. Bob222 -

    The problem was 6.4 x 10^-6. Thanks

  • Chemistry-Help Dr. Bob222 -

    Would that still equal a pH of 3.1?

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