Ka Questions

arghhhh chem propa strssing me out now!!!

ny way, i forgot the assumptions that u make when doin Ka calculations, du kno what they are?

like [H]+=[OH]-

Reply 1

silent ninja

Ka for weak acid. Assume no dissociation so moles of acid= [HA].
Secondly [H+]=[A-] so

Ka= [H+]²/[HA]

Reply 2

steve2k6

thank you!
are those the only assumptions? are there some for strong acids as well?
also what are the weak acids? everything except for H2S04??

Reply 3

hopefulloser

ethanoic acid is a weak acid, i.e. partially dissociates. HCL, H2SO4 etc are all strong acids.
I think its an assumption...but not really an assumption at our level...its something that is just done.

Reply 4

silent ninja

The definition of a strong acid is something that fully dissociates/ionises in water. So for a strong acid it goes fully to the right: HA--> H⁺ + A^-

Calculations for these are easy. For example, for pH all you need to know is H⁺.

Dont get strong acid confused with dilute, which is just a weak solution or low concentration of a strong acid.

Weak acids exist in equilibrium.

Reply 5

hopefulloser

Isn't it the calc for weak acid just [H+]2? but the str acid is the more diff one?

Reply 6

bahbahbah

yeah. for strong acids you just use the equation:
pH = -log [H+]

but for weak acids you have to use the more complex equation
Ka = [H+][A-]/[HA]
which simplifies to
Ka = [H+]2/[HA]
that 2 is a 'squared' not 'double'

for buffers you also use the equation
Ka = [H+][A-]/[HA]
this is a bit more complicated because when you do buffer calculations, [H+] is not the same as [A-] so you cant simplify it.

Reply 7

Eau

silent ninja

Ka for weak acid. Assume no dissociation so moles of acid= [HA].
Secondly [H+]=[A-] so

Ka= [H+]²/[HA]

You assume so but actually, (for informational purposes only), [H+] does not equal [A-] in solution due to the dissociation of water itself: 2H2O ----> OH^- + H3O⁺

K[H2O]² = [OH^-][H3O⁺] = Kw = 1.0 x 10^-14

Reply 8

silent ninja

Yeah i read that in the book as well, but it doesn't make sense: water does dissociate but there are as many H⁺ (or hydroxonium ions) compared to OH^- ions, so overall nothing? How does it have an affect on the acidity? My book doesn't make that clear :confused: