Formula of compounds

[mollykins]

Yes this is for homework BUT i'm not asking you to do the questions for me, i'm asking how you do them, so once i've got the method i'll work them out myself.

If you're given the wieght of three elements, that make up a compound, how do you work out the formula? I've done it for the one's that only have two elements in them but I got on this one.

Plus if you're given the the percentage (not integers) of three elements that make up a compound, how do you work out the formula? I have a feeling that it involves relative atomic mass . . .

[mollykins]

Plus on another question which i'm now stuck on, I have the amount of moles of something and the question asks for the number of ions, do i just times it by 6x10^23 ?

[mike11111]

Blimey, Molly, that's shut us all up. Like to see you get 200 replies out of this one!

[jomifl]

My chemistry is almost certainly out of date, but it works like this; you need to find the proportions of each element in the compound. you know the relative weights of the elements ( the atomic weight). If you divide the actual weight of each element by its atomic weight you will get the relative proportions of the atoms of each element. If your calculations are correct you should get a ratio of integers(for example 2:1:4 which could be sulphuric acid H2S04). You need to simplify the ratio to its lowest denominator (eg 10:5:20 would equal 2:1:4). Now you can derive your formula.

[mollykins]

For the one I last mentioned, I jsut timesed it by the avogadro constant, hoping that's right . . . .

[dundurn]

If you're given the weights of three elements, that make up a compound, then by dividing these weights by the atomic weights of the elements involved you get the proportions of the elements involved. For example if the answers you get to the divisions are 15.9, 16.1 and 32.2 then you're looking at 1:1:2.

If you're given the percentages directly then you are at the final stage given above. The figures in this case would be 25%, 25%and 50% giving you the same 1:1:2 result as before.

[mollykins]

jom i've tried that, and shouldn't I get whole numbers? but i'm not . . .

[jomifl]

Can't help you there I've forgotten that because I never needed to remember it.

[mike11111]

I was never any good at chemistry, but can't remember avocados in the lab.

[jomifl]

how close are they to whole numbers? are you using the atomic weights on the appropriate weight of element? could there be a rounding error?

[jomifl]

If you can't make sense of it post your weights and atomic weights up and I'll see if it works through.

[blackeyed]

i think there is one more step to do after dividing the given masses by their atomic masses you will get some three values; one for each of the elements now, divide the smallest of these with each of the three values this will give you the empirical formula for the compound.
and you are right about using the avogadro's number.
and is this an o level question?

[mollykins]

One of them is; 1.02g of vanadium and 2.84g of chlorine, but because they're in groups 2 and 7 wouldn't it just be VCl?

The others are; 1) 1.38g of sodium and 0.96g of sulfur and 1.92g of oxygen.
2) 34.6% copper, 30.5% iron and 34.9% sulfur
3) 2.04% hydrogen and 32.65% sulfur and 65.31% oxygen
4) 52.18% carbon and 13.04% hydrogen and 34.78% oxygen

Thanks for confirming the use of avogadros constant, it's the only thing i could think of to do with the numbers.

AS level.

[mollykins]

that might be it blackeyed, although that teacher doesn't know we've done about empirical formulas so would he give us questions to do that involved them?

[squarebear]

This is bull

I thought you were from England?

[mike11111]

"52.18% carbon and 13.04% hydrogen and 34.78% oxygen"
Fancy a sniff of that!

[mollykins]

I am, but i've gotten used to spelling sulfur like that because it's in the textbooks like that.

[Vagrant]

If you're from England, than it's " I've got" rather than "I've gotten".
However, I'm at a loss to explain the past tense of "forget"

[blackeyed]

There are some questions of these sort where you need to have knowledge about the empirical formula but not all of them need it.Your question must be one of those.
Let me know if you get the answer.

[blackeyed]

and no problem!

[gen2]

Surely, multiplying the number of moles by Avagadro's number will give you the number of MOLECULES, not ions. You will have to decide for yourself how many ions each molecule will generate and multiply again by that number.
eg each molecule of HCl will generate 2 ions and H2SO4 will generate 3.

Be aware though that not every substance will dissociate 100% into its respective ions, Please consider whether your teacher expects you to take that into consideration.

Also, the degree of ionisation will depend on the environment. Is it aqueous?