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Titrating a mixture of strong and weak acids will yield endpoints such that the volume of base required to reach each endpoint will be different for each acid, e.g. in a mixture of HCl and CH3COOH, the volume of base required to reach the HCl endpoint will be less than that for the CH3COOH endpoint. This is all down to relative acid strengths.

However, the above isn't observed when titrating a polyprotic acid, as each endpoint requires the same volume of base; this is counter-intuitive when considering the above paragraph. For example, H2SO4 also has two endpoints – one for H2SO4 and one for HSO4- – but the volume of base required to reach them remains constant. This doesn't sit right with me, considering that HSO4- is a weaker acid than H2SO4.

So my question is: why does the volume of base required to reach each endpoint in a polyprotic acid titration remain constant, even though each endpoint will result in a weaker acid? Surely the volume of base will increase each time, like is the case in a mixture of stong and weak acids?

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  • $\begingroup$ ** e. in a mixture of HCl and CH3COOH, the volume of base required to reach the HCl endpoint will be less than that for the CH3COOH endpoint **. kindly illustrate the example by using numbers and calculations $\endgroup$ – Adnan AL-Amleh Mar 13 at 11:01
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I will start with stating the obvious: in a solution of a polyprotic acid like sulfuric acid, you will ALWAYS have equal amount of each acid (proton). As you say: a molecule of sulfuric acid dissociates in two steps, and the amount of base needed to titrate the protons released at each step must necessarily be the same. Off course, if two different acids are mixed, you can have any ratio.

If you titrate a mixture of HCl and Acetic acid, the first equivalence point reached gives the amount of HCl. As HCl is by far the strongest acid, it will react first with the added base. Thereafter, the base will start to react with Acetic acid until you reach its equivalence point. The volume of base from zero to first eqv. point tells you how much HCl there is, but it is the amount used BETWEEN the first and second eqv. point that tells you how much Acetic acid you have.

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