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Homework Help: Rainwater pH with SO2

  1. Oct 10, 2009 #1
    1. The problem statement, all variables and given/known data
    Calculate the pH of rainwater in equilibrium with SO2 in a polluted air mass for which
    the sulfur dioxide concentration is 1ppm.

    SO2(g) +H2O(g) = H2SO3(aq) Kh=1M/atm
    H2SO3 = HSO3- + H+ Ka=1.7x10-2M

    2. Relevant equations

    3. The attempt at a solution

    I'm pretty lost for this one.

    I converted SO2 1 ppm to 1E-6 atm. Set up EQ expression but have two unknowns.

    1 M/atm = (1E-6 atm)[H2O] / [H2SO3]

    I don't know where to go from here. Even if I figure can calculate H2SO3 I'll still have two unknowns in second reaction. I don't think water can be 1 M as it is not a liquid. Please can someone give me a suggestion where to start.
    Last edited: Oct 10, 2009
  2. jcsd
  3. Oct 11, 2009 #2


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    Staff: Mentor

    First stage is to apply Henry's law to calculate concentration of acid.

    Next stage is to calculate equilibrium of acid dissociation - you are given only Ka1, so you are probably expected to ignore fact that acid is diprotic.

    Your unknowns in the second equation are combined by the reaction stoichiometry.

    Try calculation of pH of a weak acid or a weak base or google ICE table.

  4. Oct 11, 2009 #3
    Oh yeah, guess the acid dissociation stuff slipped my mind. I haven't done much of henry's law but looking online does this go in the right direction?

    I found pressure of water at 25 C to be 0.032 atm.

    An equation showed me P[SO2] = (1 - 0.032 atm)(1E-6) = 9.7E-7 atm
    1E-6 is 1 ppm of SO2

    Then using henry's law constant

    [SO2(aq)] = (1M/atm)(9.7E-7 atm) = 9.7E-7 M

    I am not sure how to calculate acid from this though.

    Once I get H2SO3 it is just Ka = [H+]2 / [H2SO3] correct?
  5. Oct 12, 2009 #4


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    Staff: Mentor

    Not sure if that step is necessary. You are not told it is 1 ppm of dry air.

    That may work if assumption that [HSO3-]=[H+] holds. You should check it after calculating pH.

  6. Oct 12, 2009 #5
    How do I take into account the water in the reaction then? Or how do I convert SO2 concentration to acid? That's what I don't I know how to do?
  7. Oct 12, 2009 #6


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    Staff: Mentor

    Assume total concentration of acid is that of dissolved SO2.

    Otherwise - calculating pH is a rather wide subject:

    lectures on pH calculation.
  8. Oct 12, 2009 #7
    So would H2SO3 be 1E-6 M? Using that and doing the quadratic equation to calculate x I get 1E-6 and I don't think my answer makes sense. As when I check my answer I cannot get the same Ka value reported.
  9. Oct 13, 2009 #8


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    Staff: Mentor

    pH 6 looks OK to me.

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