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Calculating pH of a Weak Acid

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    Could somebody please answer this question for me and show me your working so I can hopefully see how it's done? I'm fine with strong acids that completely ionise but I'm struggling with weak acids.



    Calculate the pH of 0.01 mol.dm-3 propanoic acid (a weak acid, which partially ionises and has a dissociation constant of 1.34 x 10-5 mol.dm-3) at 298K.
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    (Original post by TFish89)
    Could somebody please answer this question for me and show me your working so I can hopefully see how it's done? I'm fine with strong acids that completely ionise but I'm struggling with weak acids.

    Calculate the pH of 0.01 mol.dm-3 propanoic acid (a weak acid, which partially ionises and has a dissociation constant of 1.34 x 10-5 mol.dm-3) at 298K.
    Do you know the formula for the dissociation of a weak acid?

    ka = [H+][A-]/[HA]

    assume [HA] is constant
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    (Original post by charco)
    Do you know the formula for the dissociation of a weak acid?

    ka = [H+][A-]/[HA]

    assume [HA] is constant
    This plus the fact that [H+]=[A-]

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Updated: June 28, 2012
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