Chemistry Is pH of strong acid strong base titration always 7 at stoichiometric point?

[zenterix]

Homework Statement

When we titrate a strong acid with a strong base or vice-versa it is always true that the stoichiometric point occurs at pH of 7?

Relevant Equations

My understanding is that it will always be about 7.

The reason is that at the stoichiometric point the entirety of the analyte has been neutralized and the solution contains only the salt from the neutralization plus the ions from water autoprotolysis.

The salt comes from the conjugate acid of the strong base and the conjugate base of the strong acid.

These conjugates are very weak and don't change the pH of the solution much.

Thus, the pH is due to autoprotolysis of water and is thus always 7 (at 25 degrees Celsius).

 

[Mayhem]

Yes it will be 7. The reason why weak acid titration with strong base yields a pH > 7 is because the conjugate base indeed is basic enough to affect pH. The inverse is true for weak base and strong acid.

 

[Borek]

In general - yes. But...

1. "Strong acids" are often less strong than people think. Nitric acid and phosphoric acid for example, or even HSO₄^- (check pKa values).

2. The higher the ionic strength of the solution, the higher the difference between 7 and end point (nothing to do with acid hydrolysis, it happens for every solution and makes equilibrium calculations more difficult than they seem). At which point pH differs from 7 to no longer be "around 7" is left as a choice for the reader.