Consider these three titrations: (i) the titration of 25.0 mL of a 0.100 M monoprotic weak acid with 0.100 M NaOH (ii) the titration of 25.0 mL of a 0.100 M diprotic weak acid with 0.100 M NaOH (iii) the titration of 25.0 mL of a 0.100 M strong acid with 0.100 M NaOH. Which statement is most likely to be true?

a. All three titrations require the same volume of NaOH to reach their first equivalence point.
b. All three titrations have the same initial pH.
c. All three titrations have the same pH at their first equivalence point.

a. All three titrations require the same volume of NaOH to reach their first equivalence point.  

Explanation:

The concentrations and volumes of the acid are the same. Also, the concentration of the base is the same for all cases.  

To reach equivalence point, we need to have the same number of moles of the acid and base. Take note that we are only concerned with the first equivalence point. So the diprotic acid having a second dissociation is not of concern in this question.

Now the moles is equal to concentration multiplied by the volume (mole = MV) so this means that the acids have the same moles also.  

Now for a base with the same concentration, the only way to get the same moles out of it is to have the same volume.