How do I know that an acid and conjugate base are present in equimolar amounts?

Acetylsalicylic acid is a molecule in several popular drugs such as Magnyl and aspirin. It has a $\mathrm{p}K\_\mathrm{a}$ of 3.5.


I'm about to calculate the $\mathrm{pH}$ of a $\pu{0.010 M}$ solution of acetylsalicylic acid.


I've read up on how to do this and in [one video](https://www.youtube.com/watch?v=am5EWsbdvFI) the author talks about "equimolar amounts".
He says that now that we have equimolar amounts, we can write: $$\ce{\frac{[H+][A-]}{[HA]}} = \ce{\frac{[H+]^2}{[HA]}}$$


Which makes sense as there are equally many $\ce{H}$ and $\ce{A}$ ions. However when can I assume that an acid disassociated equally? Also, I thought that when it did this then it was called a strong acid? Why would I be using this formula at all for a strong acid?


If I can assume that acetylsalicylic acid disassociates into equal acid and base ions, then I can calculate my problem, but I'm skeptical as to assuming this is true.





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> I'm really confused here!
> 
> 
> 


**No, you are not!** You only (temporarily) mixed up two things, both of with you have already understood separately.


$$\ce{HA + H2O <=> H3O+ + A-}$$


**1. Position of the equilibrium**


This is described by the equilibrium constant $K$, which is the quotient of the rate constants for the forward and the back reaction: $K = \frac{k\_{forward}}{k\_{back}}$. For strong acids, of which you know that they completely dissociate, the equilibrium is on the product side.


**2. Equimolarity**


There's nothing out of nothing! If **one molecule** of $\ce{HA}$ dissociates is water, there **exactly one molecule** $\ce{H3O+}$ and **exactly one molecule** of $\ce{A-}$ generated. Not more, not less. 


Q: How could another molecule of $\ce{A-}$ appear out of nothing?


A: It can't and it doesn't happen - not even in the LEGO universe.


![deprotonation in lego](https://i.stack.imgur.com/m1Eds.jpg)


**It is just a proton transfer reaction from $\ce{HA}$ to $\ce{H2O}$; the overall number of atoms does not change!**


If is however possible that some $\ce{A-}$ may come from a different source, just by adding a salt $\ce{MA}$ of the conjugate base (think sodium acetylsalicylate) to the solution of your acid. **jerepierre has mentioned this in his comment!** But for the moment, this is another story.


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