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Topic: Law of Definite Composition  (Read 13664 times)

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Offline aaa777

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Law of Definite Composition
« on: April 21, 2008, 06:53:45 AM »
once again, i don't know where to start in solving this problem. i'm hoping you could give the steps or at least the first one?

Given:
A compound of the element A and oxygen has a mole ration of A:O = 2:3. If 8.0 grams of the oxide contains 2.4 grams of the oxygen, what is the atomic weight of A?

i'll be waiting for your reply. thank you very much.:)

Offline Arkcon

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Re: Law of Definite Composition
« Reply #1 on: April 21, 2008, 07:31:33 AM »
When we write a compound, like H2O or Fe2O3, we use a set of units, that are understood (that is to say implied and not mentioned.)  What are those units?  Hint: They're mentioned in your problem, elsewhere.  Later in your problem, you're given the amount of oxygen, in grams.  You simple have to convert the units.  Well, convert units, and understand what I just said, to know why we convert units.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline AWK

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Re: Law of Definite Composition
« Reply #2 on: April 21, 2008, 07:52:21 AM »
Reascale problem from 2.4 g to 3 x 16 g (times 20)
AWK

Offline aaa777

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Re: Law of Definite Composition
« Reply #3 on: April 21, 2008, 08:04:06 AM »
When we write a compound, like H2O or Fe2O3, we use a set of units, that are understood (that is to say implied and not mentioned.)  What are those units?  Hint: They're mentioned in your problem, elsewhere.  Later in your problem, you're given the amount of oxygen, in grams.  You simple have to convert the units.  Well, convert units, and understand what I just said, to know why we convert units.

it's atomic mass unit right? what happens after converting 2.4 grams to amu? how do i get the atomic weight of element A from there? i believe the 8 grams that was given is not the formula weight of the compound. right?

Offline aaa777

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Re: Law of Definite Composition
« Reply #4 on: April 21, 2008, 08:04:50 AM »
Reascale problem from 2.4 g to 3 x 16 g (times 20)

where did 20 come from? why do i need to do this step?


Offline Arkcon

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Re: Law of Definite Composition
« Reply #5 on: April 21, 2008, 09:01:49 AM »
what happens after converting 2.4 grams to amu?

You'll have to determine yourself, what that conversion means. So you'll be able to do it next time on your own.

Quote
how do i get the atomic weight of element A from there? i believe the 8 grams that was given is not the formula weight of the compound. right?

That's correct, you have more than one conversion to do
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline AWK

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Re: Law of Definite Composition
« Reply #6 on: April 21, 2008, 09:38:34 AM »
Reascale problem from 2.4 g to 3 x 16 g (times 20)

where did 20 come from? why do i need to do this step?


Your compound contains 3 oxygen atoms which corresponds to 3x16 = 48 grams od oxygen. In your 8 grams sample you have 2.4 grams of oxygens - divide 48 by 2.4
AWK

Offline aaa777

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Re: Law of Definite Composition
« Reply #7 on: April 21, 2008, 09:50:36 AM »
what happens after converting 2.4 grams to amu?

You'll have to determine yourself, what that conversion means. So you'll be able to do it next time on your own.

Quote
how do i get the atomic weight of element A from there? i believe the 8 grams that was given is not the formula weight of the compound. right?

That's correct, you have more than one conversion to do


i'm sorry but i can't really figure out what to do from there. i did the conversions of both the 8 gram sample of the compound and the 2.4 gram oxygen but i don't see how that will lead me to the atomic weight of element A. more hints?

Offline aaa777

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Re: Law of Definite Composition
« Reply #8 on: April 21, 2008, 09:51:50 AM »
Reascale problem from 2.4 g to 3 x 16 g (times 20)

where did 20 come from? why do i need to do this step?


Your compound contains 3 oxygen atoms which corresponds to 3x16 = 48 grams od oxygen. In your 8 grams sample you have 2.4 grams of oxygens - divide 48 by 2.4

are these steps the same with what arckon's giving? could you please elaborate why i need to perform these?

Offline Arkcon

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Re: Law of Definite Composition
« Reply #9 on: April 21, 2008, 10:03:29 AM »

Quote
are these steps the same with what arckon's giving? could you please elaborate why i need to perform these?

No, I'm trying to be more subtle, and general, so you can solve problems other than this specific one.  Follow AWK:'s advice this time, then try to see where I was coming from, so you can do the next problem.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline DrCMS

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Re: Law of Definite Composition
« Reply #10 on: April 21, 2008, 10:19:16 AM »
You have a sample A oxide with a ratio of 2:3 i.e. A2O3.

8g of A2O3 contains 2.4g of O.

How many grams of A are in 8g of A2O3?

2.4g of oxygen is how many moles?

If there are that many moles of O in 8g of A2O3 how many moles of A are there?

Once you have the number of moles and the mass in grams of A you can work out the molecular mass. 
 

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