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chemistry

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COMMON ION EFFECT
1.) What is the pH of a solution that is 0.30M in HCOOH and 0.52M in HCOOK.
Ka of HCOOH=1.7x10^-4.
2.) Calculate the pH of a buffer system containing 1.0M CH3COOH and 1.0M CH3COONa. Ka of CH3COOH=1.8x10^-5.

  • chemistry -

    1)
    I notice that your value for Ka in the question is missing units, were these missing in the original question?

    Start from the equation for the equilibrium reaction

    HCOOH<->H+ + HCOO-

    at start
    0.30M

    at equilibrium if [H+]=x

    0.30-x x x

    there is also 0.52M HCOO-

    so at equilibrium

    0.30-x x x+0.52

    Ka=[H+][HCOO-]/[HCOOH]

    Ka=(x)(x+0.52)/(0.30-x)=1.7x10^-4

    (x)(x+0.52)/(0.30-x)=1.7x10^-4


    You can either solve the quadratic or we can say that if x is small with respect 0.52 abd 0.3, we can rewrite the expression as

    (x)(0.52)/(0.30)=1.7x10^-4

    and find x

    pH is then -log (x/mol litre^-1)

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