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Titration Theory Help (1 Viewer)

andrew29223

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Can someone please help me out, getting confused with something I've seen pop up in multiple choice. Lets say we are doing a titration with NaOH with either HCl, CH3COOH or citric acid. Am I right in assuming that the same volume of NaOH will be required to reach the equivalence point for HCl and acetic acid if they have the same concentration? (Because they are both monoprotic?). And is more base required to meet the equivalence point with the citric acid because it is triprotic? Even though it is weaker than HCL? Can someone explain this with reference to equivalence point and extent of ionisation? Helpp :confused2:
 

lunaretta

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The thing to remember about titration is that the amount of acid/base required is based purely on balancing equations. It has nothing to do with how strong or weak an acid is or the degree of ionization. (or as my teacher used to put it, it doesn't matter if they are strong or weak, the H+ are still there and they need to be neutralized by an equivalent amount of OH-) So yes, since acetic acid and HCl are both monoprotic, the same volume of NaOH will be required to reach the equivalence point if they have the same concentration. You can see this easily through equation:

CH3COOH + NaOH -> CH3COONa + H2O
HCl + NaCl -> NaOH + H2O

Similarly for citric acid: C6H8O7 + 3NaOH -> C6H5O7Na3 + 3H2O. Since citric acid has 3x the amount of H+ as HCl/acetic, it comes to reason that 3x the amount of NaOH is required to neutralize it if all acids are of the same concentration.

But where does the degree of ionisation play in this? Remember that it DOES NOT matter in the volume of acid/base. Degree of ionisation matters in the selection of the indicator for titrations. Remember that you have to pick an indicator whose range occurs in within the equivalence point region of the titration being performed.

From the relationship between conjugate acids and bases, the conjugate of a strong acid is an extremely weak base (similarly for a strong base) and the conjugate of a weak acid is a weak based (similarly for a weak base). So using the HCl/acetic examples:

CH3COOH + NaOH -> CH3COONa + H2O
weak acid + strong base-> weak base + extremely weak acid. the equivalence point is a weak base (i.e. pH > 7) so phenolphthalein with the range of 8.2-10 is appropriate.

HCl + NaCl -> NaOH + H2O
strong acid + strong base +> extremely weak base + extremely weak acid. the pH of the solution (around 7) means that something like bromothymol blue or litmus is most appropriate, although you will sometimes see phenolphthalein being used.
 

someth1ng

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As above, strength of an acid means nothing for titration - it really only affects the speed of the reaction.
 

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