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Calculate the pH of a 0.10 M NaHX salt solution. Ka (H2X) = 2.1 x 10-2 Ka (HX-) = 7.3 x 10-8



Calculate the pH of a 0.10 M NaHX salt solution. Ka (H2X) = 9 x 10-5 Ka (HX-) = 5.3 x 10-11

  • chemistry -

    #1 I believe is just the pH of the salt acting as an acid.
    ..............HX^- ==> H^+ + X^2-
    initial.......0.1......0......0
    change........-x.......x.......x
    equil.......0.1-x......x.......x

    Set up k1 and solve for x. You will not be able to solve without using a quadratic equation.

    4). This one looks amazingly like NaHCO3 for which (H^+) = sqrt(k1k2)

  • chemistry -

    DrBob222 don't we have to consider the hydrolysis of HX- here?

    H2O(l)+ HX-(aq)<==>H2X(aq)+OH-(aq)-->(1)

    equ. (0.1-x) x x
    (moldm-3)

    Kh= [H2X][OH-]/[HX-]
    Kh=kw/ka(HX-)

    But we are not given the kW value and we can take it as 14 only if the temperature is 25°C.

    Let's assume the temperature is 25°C and kW=14,which gives kh= 10^(-14)/[5.3*(10)^-9]= (1/5.3)*10^(-5) moldm-3=[x^2/(0.1-x)]

    if we take (0.1-x) is approximately 0.1,we get,
    x^2 = 0.19*(10)^-5==> 2*(10)^-5 M

    x==> 4.46*(10)^-2

    But we've been given the ka of H2X.

    H2X<===> H+(aq) + HX-(aq)->(2)
    equ. (x-y) y y
    (moldm-3)

    which gives,
    y^2 =9*(10)^-5 moldm-3 * x
    y^2 =9*4.46*(10)^-7 M

    Don't we have to consider both (1) and (2) like this to find pH

  • chemistry -

    Some corrections should be added!

    x^2==>2*(10)^-6 M
    x==>1.41*(10)^-3 M

    y^2= 9*1.41*(10)^-8 M
    y^2=12.69*(10)^-8 M
    y ==>3.56*(10)^-4 M
    [H+]=> 3.56*(10)^-4 M

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