ka and pka??? (1 Viewer)

[benthejack]

hey i was just wondering im studying a solubility section in a practise exam and i found a question which i cant find in my textbook.
it goes like this:
The weak acid HA has Ka=1.20*10^-6
Pka=5.92

Calculate the concentration of all species, other than water, that are present in a 0.05mol/L solution of the acid. Discuss the valadity of any assumptions made.


i dont have any idea what this question is talking about (i probably should have listened in class) please help!!

 

[~*HSC 4 life*~]

ka has been taken out of the syllabus

 

[benthejack]

ka and pka

cheers.. i hoped somone from here would know (im from newzealand) but since it has been taken from your syllabus i spose no-one will. but if you know any other forums i can ask ill be gratefull .... thanks
benthejack

 

[nit]

If you still haven't got your answer in a coupla days I can give it to you, along with the theory...just that right now ive got a physics hsc exam...

 

[xiao1985]

ka = acid dissociation constant... say for the followin eqlbm:

HA <---> H+ + A -
ka = [H+][A - ]/[HA]

to do this q, do the ICE table:

initial (condition) 0.05 | 0 | 0
change -x | + x | + x
eqlbm 0.05 - x | x | x

which means that x moles of acid has dissociated (tho i feel reluctant to use this word)
Ka of the eqlbm will be:
x . x / (0.05-x) = 1.2 e -6

do the quadratic, u will see x = 2.44 e -4

assumption: self ionisation of water is negligible to be taken into account, (hence initial [H+] = 0

 

[benthejack]

hey thanks a heap.. i think i get it now (kind of) although i dont think weve been taught the ICE table. but i get the geist of it.
thanks again

BENTHEJACK

 

[nit]

Darn...I wanted to post that Xiao ..yeh, pretty much. Why are you reluctant to use the term "acid dissociation" btw?

Also, just a touchy point, but ud probably want to be using a lowry bronsted acid, instead of an arrhenius one. Doesn't make a difference here naturally, but its just better to use that in any case.

On another note, Zumdahl's 5% rule is useful here.Despite its dodginess, it works in these cases...

And as for the H+ concentration being initially negligible compared to the acid concentration, you might just want to demonstrate the difference in terms of orders of magnitude between 10^-7 and 5 x 10^-2 to illustrate your argument.