ka of ethanoic acid prac qn (1 Viewer)

[Masaken]

i did a prac in class that determined the Ka of ethanoic acid, involving using ethanoic acid of accurately known molarity, getting its pH with a pH probe (2.45) and then from there calculating Ka through determining [H+], [CH3COOH] and [CH3COO-]. i ended up getting a value that was much different from the theoretical value (lower than the theoretical value). how would you account for that? would i say it was because of concentration or??

and also, apparently the same method would not work to determine Ka of hcl, hno3 and other acids. why not?

 

[=)(=]

The ionisation of hcl goes to completion therefore it will have a very high ka e.g you dont need to use ice tables etc. For HNO3 not quite sure as it is monoprotic

 

[Masaken]

The ionisation of hcl goes to completion therefore it will have a very high ka e.g you dont need to use ice tables etc. For HNO3 not quite sure as it is monoprotic

hi sorry, ice tables weren't used to determine the ka (instead just the pH equation and then go from there), but so i would just say that because of it being virtually irreversible we can't assume the Ka as the reaction isn't at equilibrium?

 

[=)(=]

yea essentially ka is 0 as the hcl ionises to completion

 

[Masaken]

yea essentially ka is 0 as the hcl ionises to completion

i see, ty

 

[wizzkids]

The first question that needs to be asked is, "Did you calibrate your pH probe in the region you expected to measure?"
The electrode potential of the glass pH electrode drifts considerably over weeks. It needs to be calibrated daily. You should use buffer solutions in the range pH3 to check its calibration. The orthophosphate / phosphoric acid buffer would be a suitable buffer.

 

[Masaken]

The first question that needs to be asked is, "Did you calibrate your pH probe in the region you expected to measure?"
The electrode potential of the glass pH electrode drifts considerably over weeks. It needs to be calibrated daily. You should use buffer solutions in the range pH3 to check its calibration. The orthophosphate / phosphoric acid buffer would be a suitable buffer.

my lab partner and i did calibrate the probe and placed it in the appropriate buffer solutions (though i believe the range was up to pH 4? how would that affect it?) additionally, the probe hadn't been used in ages, so as you said it needed to be calibrated daily, this would mean that it would also affect its measurement of the pH of the acid?

 

[wizzkids]

Ideally, you should include two buffers that span the region you are interested in. pH4 is outside the region of interest, but it is better than nothing. So were you able to adjust the slope on your pH meter so that you could reliably measure pH 4?
The expected Ka for ethanoic acid at 25 C is about 3 x 10^-5. So what value did you get?