stonecold's chem questions :)

[stonecold]

I asked this in another thread but I may as well make my own as I'm gonna have a lot of questions this year. (unfortunately)

I have quick stoich question.  It has to do with ratios.

Say I am working with Na₂CO₃, do I calculate the ratio accoding to each atom, or do I break it up into ions, for example:
Ratio of sodium to carbonate is 2:1, or is it ratio of sodium to carbon to oxygen, which is 2:1:3.

I am being asked to calculate the amount (in mol) of metal ions, of 5.0g of Na₂CO₃

I had another question where i was asked to find the conc. of iron(III) ions in 40.4g of Fe(NO₃)₃.9H₂O.
I used the ratio iron ions to nitrate ions to water, 1:3:9.  Did I screw it up.  Was i supposed to do every element on its own?

Thanks.

[longy1991]

for the sodium carbonate, do the ratio for the amount of metal ions (Na⁺) per molecule, which is 2:1. Therefore there will be twice as many mole of metal ions per mole of sodium carbonate.

n(Na₂CO₃) = m/M = 5.0/106 = 0.047 mol
Therefore, n(Na⁺) = 2 x 0.047 = 0.094 mol

For the second, in terms of what do you want the concentration? do you want mol/L, g/g, ion/molecule haha, or what?

[stonecold]

it doesn't specify, so i guess mol/L.

thanks.

[longy1991]

I looked it up, and it forms crystals in its nonahydrate form, so i don't think it can be mol/L unless it is added to water. Now i'm unsure what the question wants lol.. where did you get it?

[NE2000]

40.4 grams and what longy said suggests that it's in crystalline form. Concentration in this form is most likely g/100 g (% w/w), but I'm not totally sure.

But in general the way you would approach these questions is find the number of mole (given the mass and the molar mass) and form there use the ratio of the number of ions you need compared to the molecule to find the number of mole of the ion you are referring to. After that you may need to manipulate it further to find, say, the % grams of iron (this would be the long way, however, and in such a case you would be better off finding the relative composition of iron by mass in this hydrated ionic molecule)

[stonecold]

whoops, forgot to add that the 40.4 grams is dissolved in 1 litre of solution.

I am still unclear about the whole ratio thing. I have to find the conc. of iron ions and the conc. of nitrate ions.

I get that Fe is an ion, and NO3 is another ion, and for every one Fe ion there will be 3 NO3 ions, but how to i take into account the water.  Do I treat it as an entire molecule?

....and zomg longy!!!! 50 in methods and chem!  you're my idol. 

[vexx]

whoops, forgot to add that the 40.4 grams is dissolved in 1 litre of solution.

I am still unclear about the whole ratio thing. I have to find the conc. of iron ions and the conc. of nitrate ions.

I get that Fe is an ion, and NO3 is another ion, and for every one Fe ion there will be 3 NO3 ions, but how to i take into account the water.  Do I treat it as an entire molecule?

....and zomg longy!!!! 50 in methods and chem!  you're my idol. 

i may answer your question wrong but.. it's the same as finding the mol for each molecule

Fe(NO3)3.9H20

if you have 1mol of that entire molecule..

Thats 1 mol of the Fe(NO3)3, to a ratio of 1:9, so 9 mol of H20.
(and if you were to look into Fe and No3, it's a ratio of 1:3, so 1mol of Fe and 3mol of NO3)

if it was 1 L of solution, the amount in mol of H20 would be 9mol / 1 L, so 9 Mol L-1

was that anything to do with your question? if not, im confused with what your asking hah.

[stonecold]

ahh thanks guys, i get it.  i was having this same problem in year eleven too.  i learnt it, and i managed to forget it again.  

you've just got to multiply the number of times 'x' occurs in a molecule by the number of molecules and you get the answer.



for example, say there is 1 mol of H₂SO₄, there is 2 mol of hydrogen ions.

[Greggler]

yeah its a bit confusing that.

its one of those things in chemistry that you never really understand the whole theory behind it; but you just do it.

[longy1991]

lol thanks stonecold  , but i think a heap of work, and heaps more luck helped me a lot lol.

[stonecold]

this has me super confused

A nitroglycerine patch was dissolved in approximately 20 mL
ethanol and then diluted to 100 mL. 10.0 mL of this solution
was diluted again to 100 mL with water. 20 μL volumes of the
diluted sample and 20 μL of each of the prepared standards
of nitroglycerine were injected onto a 1.5 m GLC column and
analysed using a nitrogen-specifi c detector.

Nitroglycerine standards Peak area (mm2)
Standard 5 μg/mL                   7.2
Standard 10 μg/mL                 14.6
Standard 15 μg/mL                 22.0
Diluted sample solution            10.8

a Construct a calibration curve and determine the
concentration of nitroglycerine in the diluted sample.
b What mass of nitroglycerine was in the patch?
c The patch has a total mass of 0.50 g. What is the
percentage concentration (w/w) of nitroglycerine in the
patch?

I can't do part b.  any help much appreciated. the answer to a is 8.0 μg/mL which will hopefully make it easier.

i keep ending up with a 50,000 times dilution factor, which i'm pretty sure is wrong.
what i'm doing is saying, firstly they're taking 10mL from 100mL, so a 10 times dilution factor...
and then they take 20μL from 100mL, which is a 5000 times dilution factor,
so total dilution factor of 50,000, but i'm obviously going wrong somewhere.

edit:  dw, i got it eventually.  you have to convert the conc. to μL as well.  Then multiply by 20 for 20μL.  Then apply 50,000 times dilution factor and it's all good.

[stonecold]

hey,

does anyone know something I can use to help me interpret NMR spectrographs better?
the heinemann book does a fair average job of explaining it...

any suggestions appreciate it.  thanks. 

[stonecold]

how does peak C have 2 protons?
i sorta get that A has 2 and B has 3.....

question says find the relative number of protons in each peak set if that helps. 

[chem-nerd]

the ratio of the area under the peaks is 2:3:3 for A:B:C respectively and this indicates the number of equivalent protons are present in in each group.

Peak C has 3 protons (so if your book said 2 then it is a mistake). It is the CH₃ group next to the CH₂ group, hence why it is split into a triplet.

[stonecold]

yeah, i eventually worked that out.  thanks though. 
i can't stand it when it's wrong.  it makes me spend ages worrying why i cant get it.
i wasted so much time before i concluded it was the books fault!