The graph below is a titration curve of three different solutions. Solution I is

Question

The graph below is a titration curve of three different solutions. Solution I is titrated with a base of a lower pH. Rank the solutions in terms of the strength of the acid in the solution.

Apparantly, the correct answer is "there is no difference among the strength of the acids," however I'm not entirely sure why. The explanation states that graph II has a higher concentration of acid than graph III, but why would this shift the titration curve to the right? Shouldn't the curve just become less compressed at higher concentrations as shown in the following figure?

Or would this only be the case if the question stated explicitly that only the identity of the analyte was the same in all 3 solutsions? In other words, does this mean solution II must be a of a different concentration as well as a different identity? What significance is it that solutions II and III still have the same equivalence point? Also, I don't understand why a titrant of lower pH (i.e. in the titration of solution I) would decrease the equivalence point. Wouldn't you then need more titrant to neutralize the analyte if the pH of the titrant were lower? Can someone please thoroughly explain? Thank you!

Explanation / Answer

Try to think about it this way. If you have a higher concentration of acid, it will take more titrant to shift the pH. That's why you see graph II shifted to the right of graph III. Looking at the x-axis it is evident that it took more titrant to start changing the pH on the way to the equivalence point.

Your question about compression is not quite right. Notice that in graphs (a)-(e) the volume of titrant you use is the same for any of those species. On the other hand, they start at difference pHs which is a decent indication of acid strength. So solution II is a different concentration but the same type of acid (the equivalence point was moved so it simply took more titrant).

In fact, solutions II and III do not have the same equivalence point. The volume of titrant and therefore number of moles of titrant at the equivalence point are slightly different. Solution II took a slightly higher volume of titrant to reach the equivalence point compared to solution III.

The titrant with a lower pH means "less basic". You are right, you would expect the equivalence point to be shifted toward the right because it takes more titrant to reach the same point. However, you're not given numbers here. Notice that solution I has an equivalence point just to the right of solution III. It isn't bigger by a lot, but it is defined as a larger value.

The other thing to notice that the pH at the end of the titration is lower than solution III. This also fits with the idea that solution I was titrated with a base of lower pH. This is because once the solution is overrun by base, the pH is controlled by the concentration (i.e. pH) of the base and as stated, the pH of the titrant base is lower than in solution III.

I hope this helps a bit!

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