June 20, 2024, 01:47:19 AM
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Topic: Strong-Base Titration of Mixed Strong Monoprotic and Weak Polyprotic acids!!  (Read 8254 times)

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Offline bblue

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A mixed aqueous solution of HCl and phosphorous acid (H3PO3; Ka1 = 3.00×10-2; Ka2 = 1.66×10-7) is titrated with 0.1000-M NaOH.
The first end point (cresol purple indicator) is reached after 46.56 mL of NaOH has been added.
The second end point (thymol blue indicator) is reached after a total of 67.44 mL of NaOH has been added.

1) What quantity of HCl (units of mol or mmol) was present in the initial mixture?

2) What quantity of phosphorous acid (units of mol or mmol) was present in the initial mixture?
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For the first qn, do i treat the solution as if there was no H3PO3 and just focus on the HCl?

do the moles of base at the equivalence point equal the number of moles of acid at the equivalence point?

how would i find the moles initially since i dont have any volume?

Offline Borek

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You have to decide which acid gets neutralized when - and what happens at each end point. To do so you have to look at pH of the end points (defined by the indicator used). That will give you information on the reactions that took place at each end point. Then it is all in stoichiometry.

These pages may help:

http://www.titrations.info/acid-base-titration-phosphoric-acid

http://www.titrations.info/acid-base-titration-sodium-hydroxide-and-carbonate

http://www.titrations.info/acid-base-titration-indicators-preparation

(Warder titration page may seem off, but it discusses stoichiometry of the similar problem).
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