Q21 hsc chem exam!! (1 Viewer)

[cook E]

A 0.001 mol L−1 solution of hydrochloric acid and a 0.056 mol L−1 solution of
ethanoic acid both have a pH of 3.0.
Why do both solutions have the same pH?

Okay I put 0.001 into this equation pH = –log [H3O+] and I get a pH of 3 and I know HCl completely ionises to H+ and Cl- in aqueous solution.

But I don't get how ethanoic acid works. Like I know when i put it into that pH equation it doesn't give me the same pH and I don't know if you are supposed to write something about ethanoic acid being a weaker acid(as it is in equilibrium and the degree of ionisation is less) than HCl but it has more [H3O+] concentration 0.001 mol L -1 HCl or something.

I don't know!!
Guys help me!!

 

[ibbi00]

You don't need an equation. pH = –log [H3O+] only works for acids that have completely ionised so it does not apply to ethanoic acid. You'd have to say something like HCl is a strong acid but in this case it is diluted. So the presence of hydronium ions is quite low. Although Ethanoic acid is weak it is present in a greater concentration than that of HCl. So incidentally they ended up with the same concentration of hydronium ions resulting in both solutions having the same pH of 3.0
So Ethanoic is a weak concentrated acid (relative to HCl) that partially ionises and HCl is a strong diluted acid that completely ionises. You're on the right track.