Consider the following three solutions:(i) 0.10 M solution of a weak monoprotic acid(ii) 0.10 M solution of strong monoprotic acid(iii) 0.10 M solution of a weak diprotic acidEach solution is titrated with 0.15 M NaOH. Which of the following will be the same for all three solutions?A. the volume required to reach the final equivalence point B. the volume required to reach the first equivalence point C. the pH at the first equivalence point D. the pH at one-half the first equivalence point

Option B

Explanation:

We have the tree solutions here:

A: weak monoprotic acid HA

B: Strong monoprotic acid HA'

C: weak diprotic acid H₂A

In order to explain this, let's discart the options one by one.

C and D cannot be the correct choice. This is because the pH is an expression that is determined by the following expression:

pH = -log[H₃O⁺]

In order to be the same value for all the solutions, the concentration of hydronium should be the same for all. This will be correct if all solutions were monoprotic and strong. However this is not the case.

Solution A and C are weak acids, so this means that they have dissociation constant Ka (In the case of C, it has two Ka values because it's diprotic). So, when these solutions reach the equivalence point or half the equivalence point, the concentration of the acid do not dissociate completely in solution, instead it do this:

HA + H₂O > A⁻ + H₃O⁺   Ka

0.1                        0        0

0.1-x                     x         x

[H₃O⁺] = x

Same thing happen with C, but we can see that the concentration is not the same as the innitial concentration.

Solution B is a strong acid, so it will dissociate completely in solution, so concentration of hydronium will be the same of the initial acid, and therefore the pH cannot be the same in the three solutions.

Option A cannot be either because, we have a diprotic solution, this means this acid requires two equivalence points to reach it's final, so, it takes more time and volume to reach the end point. Therefore, it's not the correct option.

Option B is the correct one, because regardless that we have a weak or a strong acid, or if it's diprotic or triprotic, the three of them have the same concentration of acid, and to reach the first equivalence point (the only point for A and B) will be the same for solution C. Therefore, this is the only thing the three solutions have in common.

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