Titration question..

[ninbam1k]

My tuto did this question like twice, but I keep on forgetting how to do it, so I was wondering if there were any VNers who could help.

A fertiliser has 8% nitrogen by mass, found in the form of NH4+. 12.5 g of fertiliser sample was taken and dissolved into 150 ml of water. 30 ml aliquot was taken out and added to 125ml of 0.4M NaOH. 25ml aliquot of the resulting solution was taken out and titrated against 0.095M of H2SO4. Find the titre volume of H2SO4.

HELP PLEASE

[taiga]

I'm not going to answer it because I dont have the right stuff with me atm

If it is 8% by mass, then you can find the mass of nitrogen in the 12.5 grams of fertilizer. All of that nitrogen exists as NH4+, hence you can find the amount of that NH4+ as well.

If that amount is dissolved, and then 1/5th (and assuming it is dissolved equally) is taken out. You have 1/5th of that amount of NH4+ as you originally had.

Then if it is added to 0.4*0.125 mols of NaOH, then you end up with some of the base in excess. Find how much that excess is by comparing the amount of NaOH to the amount of NH4+.

Then you have the amount of excess NaOH.

If you are taking 25ml from that remaining excess, you can figure out how many mols of NaOH you are taking out.

If you know the number of mols of NaOH, then you can figure out how much 0.095M H2SO4 is needed (in ml).

I think it's 1 significant figure as well.

Sorry I couldn't do it with numbers, if you need worked answers, I can't post them until tomorrow (someone will probably beat me to it )

[m@tty]

It's actually better to show the thought process rather than do it all..

And yeah, 1 sig fig.

[thushan]

OK.

12.5 g of fertiliser.

m(N) = 12.5 x 8/100 = 1 g (accuracy: 1sf) [using exact values throughout calculation]

=> n(N) = m(N)/28.0 = 1/14.0 = 0.0714 mol (accuracy: 1sf)
=> n(NH4+) = n(N) = 0.0714 mol (accuracy: 1sf)

=> n(NH4+) [in aliquot] = 0.0714 x 30/150 = 0.0014 mol (accuracy: 1sf)
      n(NaOH) [original] = cV = 0.125 x 0.4 = 0.05 mol (accuracy: 1sf)
=> n(NaOH) [after reaction with ammonium] = 0.05 - n(NH4+) [in aliquot] = 0.0357 mol (accuracy: 1sf)

=> n(NaOH) [in second aliquot] = 0.0357 x 25/155 = 0.00576 mol (accuracy: 1sf)  
=> n(H2SO4) = 0.00576/2 = 0.00288 mol (accuracy: 1sf)  
=>V(H2SO4) = 0.00288/0.095 = 0.0303 L = 0.03 L.

Hence, the titre volume is 0.03 L.


Edited.

[taiga]

It's actually better to show the thought process rather than do it all..

And yeah, 1 sig fig.

Well yeah that's why I posted xD

Just a bunch of numbers and equations can get it mixed up.

[thushan]

 

It's actually better to show the thought process rather than do it all..

And yeah, 1 sig fig.

Just a bunch of numbers and equations

Damn, just like my post.

[taiga]

It's actually better to show the thought process rather than do it all..

And yeah, 1 sig fig.

Just a bunch of numbers and equations

Damn, just like my post.

Well I scanned what you wrote and it matches my thought process, so now he has everything he needs (hopefully)

[m@tty]

It's actually better to show the thought process rather than do it all..

And yeah, 1 sig fig.

Just a bunch of numbers and equations

Damn, just like my post.

Haha, yeah I thought of that as soon as I saw what he said.

It's good that the thought process is first. And then the number crunching is there to make it all crystal clear.

[ninbam1k]

thanks guys was so dam confused by this question. I end up getting like 600mls o_o

Btw, at thushan's working out, why is the molarmass of N 28, not 14? Is it because it's N2?

[thushan]

thanks guys was so dam confused by this question. I end up getting like 600mls o_o

Btw, at thushan's working out, why is the molarmass of N 28, not 14? Is it because it's N2?

Ahh crap, that's an error. It's 14.
Edit: Fixed.

[luken93]

thanks guys was so dam confused by this question. I end up getting like 600mls o_o

Btw, at thushan's working out, why is the molarmass of N 28, not 14? Is it because it's N2?

Ahh crap, that's an error. It's 14.
Edit: Fixed.

Thushan made an error

That's it, no MEUP Chem award for you...

EDIT(m@tty): Tut tut, spelling error.. No 40 in English for you

[taiga]

Haha dog act m@tty

[luken93]

WTF, you can edit my posts now m@tty cos you're a moderator?

We're doomed hahaha


Yep. I sure can