weakbase-strong acid question

[samanthalee14adis1G]

if the sample is a weak base, and a strong acid is added, why does it reach a stochiometric point below ph7?

[Anmol Dhaliwal 2C]

The stoichiometric point is below the ph of 7 because acids have a pH lower than 7. If you add a weak base and a strong acid, the acid has a stronger affect on the stoichiometric point therefore lowering the pH below 7. Basically, after you mix the strong acid and weak base together, the solution is ends up being acidic (since the acid is stronger and the strong acid dominates the weak base) and therefore has a pH below 7 since the pH of acid's is below 7.

[samanthalee14adis1G]

but isnt that the explanation lavelle told us not to use?

[Celine_Ngo_3L]

It's because when the weak base's conjugate acid reacts with water, it creates an excess of H3O+ ions.

For example, if HCl is used to titrate NH3, the chemical equation would look like this:

HCl + NH3 + H2O--> NH4+ Cl- + H2O --> NH3 + H3O+ Cl-

NH4+ gives a proton to H2O, making more H3O+ ions in the end. Therefore, the stoichiometric point would be below 7.

Similarly in a strong base-weak acid question, the weak acid's conjugate base creates OH- ions in the end, making the stoichiometric point above 7.