Weak acid

[M Leung]

Why is CH3COOH a weak acid?
I've been searching from the internet but all the answers just said CH3COOH is not fully ionised in aqueous solution.

[goldstanda3269]

Why is CH3COOH a weak acid?
I've been searching from the internet but all the answers just said CH3COOH is not fully ionised in aqueous solution.

Knowing why or even memorising that it is a weak acid is beyond the scope of GAMSAT (ACER expects that you know what is weak or strong based on Ka and/or pKa values).

But, for your curiosity, the main issue is the stability of the conjugate base. HCl is powerful as an acid because when H+ is released Cl gains the octet as Cl- and thus has the electronic configuration of a noble gas (the most stable atoms in the periodic table). No organic acid has a conjugate base that, even with resonance, can anywhere approach the stability of a noble gas.

[M Leung]

And how do I work out the Ka/pKa of a compound in order to determine for a weak/strong acid?

[goldstanda3269]

And how do I work out the Ka/pKa of a compound in order to determine for a weak/strong acid?

Any acid that dissociates 100% into ions is called a strong acid. If it does not dissociate 100%, it is a weak acid.

A strong acid (i.e. HCl) has a Ka of, basically, infinity, and an incredibly low pKa. A weak acid is the opposite: very low Ka and high pKa.

I think it would be to your advantage to study some up-to-date General Chemistry chapters with GAMSAT-level practice questions. I have added the Gen Chem Masters Series eBook to your account for free, for 3 weeks. Chapter 6 is acids and bases. Good luck with your studies.