Titration MC question (1 Viewer)

[deswa1]

Hey guys-

Can someone please explain the working for this:

http://www4.boardofstudies.nsw.edu....wer=B&courseID=15050&testQuestionID=300268719

I'll run you through my thought process because maybe it'll become clearer where I'm wrong:

So the burette should also have been rinsed with the acid but it wasn't which means that the concentration of acid in the burette is actually lower than it should theoretically be. The conical flask shouldn't have been rinsed with base but it was which means that the concentration of base in the flask is actually higher than it should theoretically be.

So say if the theoretical titration equation thingo is meant to be:

HCl + NaOH -> Whatever (i.e. a 1:1 stoichometric relationship between the two), due to the higher base concentration and lower acid concentration, its actually something like:

0.9 (HCl) + 1.1 (NaOH) -> Whatever. Therefore the number of moles of base will be calculated to be lower than it should be and hence the base concentration is lower than it should be.

Am I looking at this in totally the wrong way?

Thanks heaps

 

[Parvee]

I would of guessed it was B as well :/

The question is Q14 from 2003.
The success one book says:
Rinsing the flask with a base will result in too many moles of base in the flask. More moles of acid will be needed to neutralise it. Rinsing the burette with water will dilute the acid. More moles of acid will be needed to neutralise the base. Therefore the number of moles of base will appear to be higher that it actually is and the calculation of its concentration will appear higher than it actually is.

 

[barbernator]

ok ive got this gimme a sec.

Firstly they rinsed with water instead of acid. This will mean that the molarity of acid in the burette will be lower than expected as it has been diluted. (i.e. pH4 instead of 3)

Pipette rinse is fine

Conical flask rinse will result in the solution having more OH ions than expected as there is already some basic solution in the flask

Hence, they will have to use MORE acid from the burette to reach the endpoint in the conical flask as there are more OH ions to neutralise and less H ions in the burette solution than expected. So they will THINK that the base is more concentrated because they had to use more moles of the acid they thought than they would have if the acid wasn't diluted and the base wasnt stronger.

 

[Aesytic]

the solution in the conical flask isn't more basic; there's just more moles of base in there, and hence you'll need more acid to neutralise all the base

 

[barbernator]

the solution in the conical flask isn't more basic; there's just more moles of base in there, and hence you'll need more acid to neutralise all the base

yep sorry that's correct.

 

[deswa1]

Oh yeah that makes sense. Thanks guys

 

[deswa4]

what a noob

 

[deswa3]

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