Question relating to Fertiliser Back Titration

[VCEMan94]

Why must the fertiliser solution be heated until all the Ammonia has turned into gas? How does it affect the titration?

[Greatness]

The ammonia can interfere with the reaction in the second stage. (So the Sodium hydroxide and hydrochloric acid reaction) Not sure on the specifics of this tho.

[VCEMan94]

yeah, but would it result in more or less hcl being required?

[Greatness]

umm im not too sure about that, but im guessing more...

[Readinya]

I found this in the answers of the Nelson Student Manual:

Why is all ammonia removed before performing titration?
Ans: If present in the solution, the NH3 is a weak base and would react with the HCl as well, so we could not precisely know the amount of NaOH remaining after the preliminary reaction.

Ques: A group of students obtained a mean value for the concentration of nitrogen in a lawn feed that was lower than the true value. Identify four possible sources of experimental uncertainty that could account for this.
Ans: (I only put the one relevant to VCEMAN's question up:) Not all of the NH3 was boiled out of solution, so some of this basic gas reacted with the HCl in the titration. Thus it appeared that more NaOH remained after the preliminary reaction.

[Asx4Life]

 - Equation for fertilizer reacting with Sodium Hydroxide: NH4(aq)+NaOH(aq)=NH3(g)+H2O(l)
 - You heat the solution to speed up the rate of reaction.
 - If there is still NH3 gas (I think you use litmus paper to check), this would mean there is still a reaction going on.
 - If the reaction is incomplete, there would be more NaOH left in excess in the flask,
   so more HCl is needed to titrate with NaOH to find the amount of NaOH in excess.
 - According to the equation n(NaOH)reacted =n(NaOH) initial - n(NaOH) excess,
   a higher value of n(NaOH) excess would give a a lower value of n(NaOH) reacted.
 - Hence an underestimation would occur

[VCEMan94]

Thanks GUys
Does the Nelson student manual say anything else helpful about the prac?

[VCEMan94]

Thanks GUys
Does the Nelson student manual say anything else helpful about the prac?

[Readinya]

Thanks GUys
Does the Nelson student manual say anything else helpful about the prac?

Quite a lot, actually. Download the student manual answers 'sam_03' from http://vce.atarnotes.com/forum/index.php/topic,23280.0.html . If you can, get a copy of the Nelson Activity Manual Prac 3.1 (not just the answers) so that you can practice the discussion questions.