VCE Chemistry Question Thread

[bae]

I'm having a bit of trouble with titration
We are trying to determine the amount of Nitrogen as ammonium in a fertiliser

We have a sample of fertiliser, weighed it and added de-ionised water to dissolve it. Once it was dissolved, sodium hydroxide was added to it and then it was heated until all the ammonium had turned into vapour. Then we added methyl red indicator and titrated it. I don't understand what titrating the solution actually does in terms of determining the Nitrogen in the fertiliser. If someone could help me that would be awesome

It's a back titration practical, so when we titrate the NaOH with HCl, this is done to find the moles of HCl. And thus work back to find out the moles of NH4(fertiliser).

This is done like this:
n(HCl) = n(NaOH) leftover, since 1:1 ratio
n(NaOH) reacted with fertiliser = n(NaOH) initial - n(NaOH) leftover

thus, n(NaOh) reacted = n(NH4)

When we know the moles of NH4, we can work out the moles of Nitrogen, and then the percentage mass of nitrogen in the fertiliser.

n(N) = n(NH4)

percentage nitrogen in fertiliser = m(N)/m(NH4) x 100

You would probably be asked to compare this with the manufacturer's statement etc.

For example, the manufacturers statement was 27% nitrogen content in the fertiliser, however it was found that such a sum has been overestimated as our results showed a 13% nitrogen content by mass.

And don't forget to include possible errors in the experiment.

[qwerty101]

Just need help interpreting this.

I can successfully apply the indicator when presented with a ph curve, however need a bit of clarification.

1) When looking at the ph curve, do we choose the indicator which changes colour across the sharp equivalence point section, i.e. when it starts to when it ends which may be 3 to 8 (ph). Rather than the middle which might be 5.5, hope that makes some sense?
2) what does acid form and base form mean? what do the different colours indicate?

Thanks

[cosine]

Can someone please explain aqueous solutions to me, having troubles getting my head around it. Thanks

[qwerty101]

Can someone please explain aqueous solutions to me, having troubles getting my head around it. Thanks

Hey mate,

An aqueous solution is a solution in which the solvent is water. That means, any solute such as anhydrous sodium hydroxide is dissolved in water to produce hydrous sodium hydrous (where water molecules are in the structure)

Denoted by (aq) - where solubility rules come into play.

[qwerty101]

I'm having a bit of trouble with titration
We are trying to determine the amount of Nitrogen as ammonium in a fertiliser

We have a sample of fertiliser, weighed it and added de-ionised water to dissolve it. Once it was dissolved, sodium hydroxide was added to it and then it was heated until all the ammonium had turned into vapour. Then we added methyl red indicator and titrated it. I don't understand what titrating the solution actually does in terms of determining the Nitrogen in the fertiliser. If someone could help me that would be awesome

We did the same prac at school. This is a back titration. Once the ammonia has been given off (NH3) you titrate it with HCl. Record the titre and use your knowledge with back titrations to find the reacting amount of moles of NaOH i believe it was.

Titrating is a process in which you have a accurately known amount of a solution (e.g. known concentration) and this is called the standard solution and is placed in the burette (usually). The volume is delivered and you calculate your average titre. This gives you concentration and volume, use this to find the mols and then use your equation to find your other substance you're investigating.

Some basic year 11 knowledge you should already be familiar with, try have a look at your year 11 book. It was something i did prior to the start of the year.

[cosine]

Hey mate,

An aqueous solution is a solution in which the solvent is water. That means, any solute such as anhydrous sodium hydroxide is dissolved in water to produce hydrous sodium hydrous (where water molecules are in the structure)

Denoted by (aq) - where solubility rules come into play.

Thanks qwerty101

So you're saying that when sodium hydroxide is dissolved into water, it will be denoted (aq). But speaking in terms of the molecules of NaOH(aq), are the Sodium atoms bonded to the hydroxide atoms, or are they seperated but found in the same solution? Thanks

[alchemy]

Ignore this.... I was wrong :/

[thushan]

Careful guys.

NaOH cannot be considered a molecule. NaOH (s) is an ionic solid, which forms a lattice as opposed to discrete "molecules".

When put into water, the Na+ and OH- separate, and act as independent ions.

NaOH (aq) is really a "lazy" way of writing Na+ (aq) + OH- (aq).

[lzxnl]

Careful guys.

NaOH cannot be considered a molecule. NaOH (s) is an ionic solid, which forms a lattice as opposed to discrete "molecules".

When put into water, the Na+ and OH- separate, and act as independent ions.

NaOH (aq) is really a "lazy" way of writing Na+ (aq) + OH- (aq).

To add to this, in VCE you wouldn't ever consider anything containing a metal to be a 'molecule'. Metallic compounds generally form ionic lattices. Only metal complexes could potentially be considered molecular species.

[cosine]

To add to this, in VCE you wouldn't ever consider anything containing a metal to be a 'molecule'. Metallic compounds generally form ionic lattices. Only metal complexes could potentially be considered molecular species.

So does (aq) mean the atoms are bonded together or are they separated? Why does everyone make it so complicated for? :/

[bae]

So does (aq) mean the atoms are bonded together or are they separated? Why does everyone make it so complicated for? :/

haha, basically an aqueous solution is a solute dissolved in the solvent (water). Anything that is labelled as aq has been dissolved in water.

[Redoxify]

haha, basically an aqueous solution is a solute dissolved in the solvent (water). Anything that is labelled as aq has been dissolved in water.

yep i agree with you

[lzxnl]

So does (aq) mean the atoms are bonded together or are they separated? Why does everyone make it so complicated for? :/

Which atoms? If you mean NaOH, this is an ionic solid and ionic solids dissociate in solution, so the ions separate.

If you mean something like HF, which is covalent molecular, then in aqueous solution it exists (mostly) as complete HF molecules hydrogen-bonded to water.
Similarly, dissolved acetone in water exists as acetone molecules that hydrogen bond to water through the polar C=O bond. The difference is these are covalent molecular, whereas NaOH isn't.

As an aside, if you dissolve K₃[Fe(CN)₆] in water, then although you do get potassium ions surrounded by water molecules, the anion in this case is a complex ion, [Fe(CN)₆]^3- and this anion really does exist in a non-dissociated form.

[qwerty101]

Could someone please check out my question above. Thanks 

Also, why do we need to find how many moles of HCl that reacted and was left? what makes you think we need to do this? this is something that wouldnt come to my mind!

[qwerty101]

How much water must be added to 100 mL of a solution of a strong base of pH 13.0 in order to decrease the pH to 11.0?

The pH of a solution formed when 20.00 mL of 0.00100 M HCl is mixed with 20.00 mL of 0.00100 M Ba(OH)2 ?

The number of ions in 5.00 g of H3PO4 is

thanks