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# Calculating pH of a Weak Acid Tweet

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1. Calculating pH of a Weak Acid
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.
2. Re: Calculating pH of a Weak Acid
(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
3. Re: Calculating pH of a Weak Acid
(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-]