[linney]

Hey, we did this lab in school and I'm stuck on this question that's suppose to be worth alot of marks. If you can answer it ASAP it would be very helpful. Thanks!

We had equal volumes of strong acid, HCl and the weaker acid CH3COOH in separate beakers. We titrated each beaker with some NaOH.
The results for our group were: 46 drops of NaOH was needed to neutralize the CH3COOH.
49 drops of NaOH was needed to neutralize the HCl

Each group in class posted their results on the board. The results varied. Some groups only needed 35 drops of NaOH to neutralize the HCl.


Question:The two acids used have vastly different Ka values. Explain why the results obtained appear to be independent of Ka values. Le Chatelier's Principle may be helpful to you in this explanation.

[syko sykes]

my guess would be because of differing concentrations. It would take about the same amount of a strong base to titrate a high concentration weak acid of a low concentration of a strong acid. Perhaps the HCl is something like .01M while the CH₃COOH is 1M.

[rctrackstar2007]

I don't understand how Le Chatelier's Principle may be helpful to you in this explanation because there isn't much of a stress on this system except for a shift in an equilibrium if one is present (I don't think one is)

But

my guess would be because of differing concentrations. It would take about the same amount of a strong base to titrate a high concentration weak acid of a low concentration of a strong acid. Perhaps the HCl is something like .01M while the CH₃COOH is 1M.

makes a lot of sense to me

Borek...

[mike]

But
Quote from: syko sykes on Today at 12:10:12 PM
my guess would be because of differing concentrations. It would take about the same amount of a strong base to titrate a high concentration weak acid of a low concentration of a strong acid. Perhaps the HCl is something like .01M while the CH3COOH is 1M.
makes a lot of sense to me

No it doesn't! The same volumes of 0.01M HCl and 1M CH3COOH would surely require different volumes of the same titrant

In this titration it is purely the number of moles of protons being neutralised by the number of moles of hydroxide.

[Borek]

No it doesn't! The same volumes of 0.01M HCl and 1M CH3COOH would surely require different volumes of the same titrant

Exactly. Besides, I don't get the original experiment. All groups have identical samples? All groups should have identical results. They have different samples, but every group had same volumes of both acids? Every group should have two identical results, different then other groups.

Assuming there is identical amount of acid (in terms of equivalents or number of moles, it will be the same in this case) you should use identical amount of base for neutralization of both to the equivalence point.

When it comes to the end point amount of base used will be different, as weak acid will either show buffer effect lowering pH or its salt will hydrolyze raising pH. But that's rather theoretical, as in the case of acetic acid difference in volume of titrant used is below 0.01%, so it is hardly detectable.

Note that equivalence point is when you have stoichiometric amounts of reagents mixed, while end point is where you detect titration end. They are close, but usually not identical.