Dr bob,
Thanks, i understand that, at last!
But wot if im asked to calculate when it is given as words? would it still be the same?
Ie. instead of Al2O3, im giving it in words ie aliminium oxide.
Basically im asked to calculate the mass of oxygen and the mass of aliminium in aliminium oxide but its given in word apose to a chemical equation?
Not sure if im explaining this corretly.
Its probably very simple, but im not getting it. Is it just to do with the valancy?
Thanks (Again!!)
do u just take them as 1?
You have two aluminum ions combined with three oxygen ions to make one molecule of aluminum oxide. If this isn't exactly what you want, please repharase the question and provide more data and I'll try again.
will this always be the case for aliminium oxide?
Wot if it was oxide of arsenic? arsenic having a valency of 5 and oxygen of -2?
As is +5.
O is -2.
The common denominator of 5 and 2 is 10.
So we take 2 As @ +5 each to make +10.
And 5 O @ -2 each to make -10.
+10 and -10 = 0
The formula is As2O5. And it always works for the simplest empirical formula. (It doesn' work in a very few cases, for example it doesn't explain why mercury(I) chloride is Hg2Cl2 and not HgCl.) There is an easier way to do this, now that you see the principle involved. You can always fall back on this as a back up but here is the easy way.
You want the formula for arsencic oxide.
Write the symbols. AsO
Now, in your mind, (and it doesn't hurt at the beginning to write small numbers to help remind you), write 5 over the As and 2 (forget the signs) over the O.
Now, in your mind again, draw an X. One leg of the x starts at the number 5 you have written over the As and goes to the bottom of the O. The other leg of the x goes from the 2 over the O to the bottom of the As (between the As and O). Now just place the top numbers at the bottom of the X. The 5 goes to the bottom of its leg so the subscript for O is 5. The 2 over the O goes to the bottom of its leg so the subscript for As is 2. And the formula is As2O5. Simple, huh? I have seen this called the "criss-cross method". The only thing you must remember with this shortcut is (in most cases but not all of them) you sometimes end up with multiples of the formula. For example, Barium oxide is Ba +2 and O -2. Doing it the equal sign way we see immediately that BaO is the formula because +2 and - 2 = 0. Doing it the X way, gives us Ba2O2; therefore, when we take the shortcut, just remember that we usually want the SIMPLEST empirical formula. Therefore, we see that we can divide the 2s by 2 to give us 1 and the formula is BaO. Let me know if there is anything about this you don't understand. Chemistry really isn't hard; we just must be disciplined enough to follow the rules.
Thanks,
the criss cross method is v good!!
Thanks a million! :)