Chemistry 3/4 2013 Thread

[Limista]

TLC defs works for non-polar solvents. Dw about paper chrom, not on study design.

If I was to use a polar solvent during the TLC process, how would this affect the chromatogram compared to when I was using a non-polar solvent?

[LOLs99]

yes

Thanks. No more confusion                                                                                                                                                                                

[teletubbies_95]

This thread has been quiet!
I 've been revising moles again and I came accross this question

An analysis is carried out on a sample of unknown gas . The density of gas is 2.86 grams per litre at STP. The molecular formula of the gas is :
A) HCl b) Cl2 c) NO2 D) SO2

i know that i have to substitute the density information using the pv=nrt , but the problem is that I dont know how to derive it ?
OMG , this is such a stupid question , and thanks

[thushan]

Derive pV = nRT?

Boyle's Law shows that pV = constant (at constant temperature, and we are talking about a fixed amount of gas in an expandable container, so constant amount)

Another experiment showed that pressure and temperature are proportional.

A third showed that the amount of gas and the volume are proportional.

Combine these facts, and you end up with pV = k x nT, where k is a constant. That constant happens to be 8.31 (also known as R).

[teletubbies_95]

sos. i worded my question wrong. in other words , how do you include the density into the pv=nrt formula?

[Mr Keshy]

Also, thanks for the question

[thushan]

sos. i worded my question wrong. in other words , how do you include the density into the pv=nrt formula?

It gives you the fact that 1 L of gas would weigh 2.86 g. 1 L of gas would weigh 2.86 g. Does this help at all? Perhaps...what if you determined the amount of gas that would occupy 1 L?

[KevinooBz]

You have the density as 2.86g/L. So you need to find M(gas). One mole of gas at STP occupies 22.4L. You multiply 2.86 by 22.4 and you get the answer which should be d)SO2.

[teletubbies_95]

Hey thanx guys!
I learnt how to do it ! by subbing in density and n=m/mr formulas , i got Mr= dRT/p . just got confused !
And woah, thanks kevinooBz  , i didnt think of it that way!
Thanks thush !
Kesh...ahah...very funny !

[teletubbies_95]

What way should I write answers in chem using scientific notation or no scientific notation?

[Mr Keshy]

Thanks for the question!

I personally use sci notation for anything more than 3 decimal places

I'm also curious actually, I don't know if I'm getting into a good or bad habit.

[teletubbies_95]

I use sci notation for every calculation. I was just wondering if thats acceptable or not ? or is it better if we use no sci notation!

Thanks for answering

[Mr Keshy]

I use sci notation for every calculation. I was just wondering if thats acceptable or not ? or is it better if we use no sci notation!

Thanks for answering

Thanks for your opinion. But you're wrong.

That actually seems like a better idea. But I ceebs writing 1x10^-1 instead of 0.1

Generally, my laziness kicks in. There are 6 different characters, therefore, there should be at least 4 characters for me to bother using sci notation.

[Conic]

pV = k x RT, where k is a constant. That constant happens to be 8.31 (also known as R).

Shouldn't that be : PV = k x nT
and then :             PV = nRT
Since R=k?

[thushan]

Shouldn't that be : PV = k x nT
and then :             PV = nRT
Since R=k?

Ah my bad, careless mistake, thanks!

Fixed.