Tiration of weak acid with strong base

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Tiration of weak acid with strong base

Postby Chem_Mod » Sun Aug 21, 2011 12:53 pm

Question: I have a question regarding titrations of a weak acid with a strong base. As an example, we had a known molarity of NaOH (titrant) reacting with HCOOH of a specific molarity and volume. I understand that the # of moles of HCOOH=# moles of NaOH=# moles of HCOO- (conj base of HCOOH).

However, the molarity of the HCOO- base was taken from the number of moles of NaOH, but I was thinking that #moles of HCOOH should equal the #moles of NaOH (which in turn should equal the number of moles of HCOO-).

Chem_Mod
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Re: Tiration of weak acid with strong base

Postby Chem_Mod » Sun Aug 21, 2011 12:54 pm

Answer: In the example I did we calculated the pH at the stoichiometric point of the titration of 25.00 ml of 0.100 M HCOOH(aq) with 0.150 M NaOH(aq). Note that we have a weak acid and a strong base so the stoichiometricpoint will be above pH 7.

AT STOICHIOMETRIC POINT: # MOLES OH- ADDED = # MOLES ACID IN SAMPLE
STOICHIOMETRIC POINT SALT AND WATER IN EQUILIBRIUM:
Na+(aq) + HCOO-(aq) + H2O(l) ---> HCOOH (aq) + OH-(aq) + Na+(aq)
Need to calculate [OH-] and convert to [H+] to obtain pH. Let [OH-] = X and set up equilibrium table.


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