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Homework Help: PH and the Acid Dissociation Constant

  1. Mar 5, 2006 #1
    1. What is the hydronium ion concentration and the pH for an aqueous solution of NH3 that has a hydroxide ion concentration of 2.25E-3 M?

    a. 4.44 x 10-11 M, 3.65
    b. 4.44 x 10-11 M, 10.35
    c. 4.44 x 10-12 M, 2.65
    d. 4.44 x 10-12 M, 11.35

    [OH] = 2.25E-3
    pOH = -log(2.25E-3) = 2.6478
    -log[H3O+] = 14 - 2.6478 = 11.352 = pH
    [H3O+] = antilog(-11.352] = 4.44E-12


    2. Determine the acid dissociation constant for a 0.010 M nitrous acid solution that has a pH of 2.70. Nitrous acid is a weak monoprotic acid and the equilibrium equation of interest is HNO2 + H2O <-> H3O+ + NO2-.

    a. 8.0 x 10-3
    b. 2.0 x 10-3
    c. 5.0 x 10-4
    d. 4.0 x 10-4

    K_a = [NO2-][H30+]/[HNO2]

    pH = -log[H3O+]
    2.70 = -log[H3O+]
    [H3O+] = antilog(-2.70) = 0.001995 M

    K_a = [0.001995 M]^2/[0.010 M - 0.001995 M] = 4.97E-4

    Thanks.
     
  2. jcsd
  3. Mar 5, 2006 #2

    Borek

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    Staff: Mentor

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