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One gram of an element M combines with 3.9 g of chlorine. It forms an oxide with the formula M2O3. What is the element?

a. Li

b. Fe

c. Al

d. B

e. Au

What I'm confused about is how the grams impact the problem. I tried doing it without considering the grams and got it wrong, so I know they do something.

Then I tried it like this. Chlorine has an oxidation of -1 and oxygen is -2 and because there are 3 oxygen atoms in the molecule, that means that the total charge on the oxygen is -6 which means that M must have a charge of +3. Aluminum is the only choice that has a charge of +3, but then what is the point of the first part of the sentence?

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Iron can have a 3+ oxidation state as well. You've certainly whittled down your choices by looking at the charge, but you will need to now work out the molar mass of M to figure out exactly what it is. Since you know the oxidation state of M, you should be able to figure out how many chloride ions it bonds to. Knowing that 1 g of M combines with 3.9 g of chlorine and knowing the ratio of Cl:M, you should be able to figure out 1.) how many moles of M are in the 1 g and therefore, 2.) the molar mass of M.

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So M bonds to 3 cloride ions (MCl3), but how do I figure out how many moles of M are in the 1 g?

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1 mol (6.022141*10^23 particles or molecules) of Chlorine has ~35.5 grams (stable isotopes with 35u ~75% and 37u ~25% abundance).

So 3.9 g / 35.5 g = 0.11 * 6.022141e+23 = ~6.6*10^22 particles.

6.6*10^22 particles / 3 Chlorine in each molecule of MCl3 = ~2.2*10^22 molecules of MCl3

1 g / 2.2*10^22 = ~4.545*10^-23 g per particle of M

Divide it by 1 u = ~1.66*10^-24 g

4.534*10^-23 g / 1.66*10^-24 g = ~27u

Aluminum has ~27u mass.

Aluminum is the only choice that has a charge of +3,

Aluminum has charge +13 in nucleus (and 2+8+3 electrons = -13 charge)
Boron has +5 in nucleus (and 2+3 electrons = -5 charge)
http://en.wikipedia.org/wiki/Boron
Either of them has 3 valence electrons.

Edited by Sensei

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A few things.

1. Why in Earth are you converting to number of molecules? Stay in moles when doing these questions unless it specifically asks for an answer in number of molecules. You're making the question unnecessarily complicated.

2. Aluminium does not haves charge of +13, nor boron a charge of +5. Simply adding up the electrons like that does not tell you the charge.

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Thanks Sensei!

hypervalent_iodine How would I do it while in staying in moles?

So I did

3.9 g Cl x 1 mol Cl/35.45 g Cl = .11 mol Cl

Because the formula unit is MCl3,

.11 mol Cl x 1 mol M/3 mol Cl = .0367 mol M

But then what do I do from there to get the mass of M?

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1. Why in Earth are you converting to number of molecules? Stay in moles when doing these questions unless it specifically asks for an answer in number of molecules. You're making the question unnecessarily complicated.

At least author will learn how to calculate number of particles and mass of single particle or molecule.

2. Aluminium does not haves charge of +13, nor boron a charge of +5. Simply adding up the electrons like that does not tell you the charge.

Aluminum has 13 protons in nucleus and 13 electrons.

Boron has 5 protons in nucleus and 5 electrons.

It's obvious that when number of protons is equal to number of electrons they cancel their charges and we have neutral element.

Edited by Sensei

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At least author will learn how to calculate number of particles and mass of single particle or molecule. Aluminum has 13 protons in nucleus and 13 electrons.http://en.wikipedia.org/wiki/Aluminium Boron has 5 protons in nucleus and 5 electrons.http://en.wikipedia.org/wiki/Boron If it's obvious that when number of protons is equal to number of electrons they cancel their charges and we have neutral element.

Right, but these are ionic species and they do have charges, just not the charges you mentioned in your post.

As for the molecules bit, the OP appears to have a fairly basic understanding of chemistry and while what you brought up is also basic, bringing it in the way you did is extremely unhelpful to them and probably quite confusing. It's best to save that stuff for when they actually have questions about it and let them answer questions using methods that are simple and that make sense. There is no reason to convert to number of molecules as you have done and it creates more work than is needed to answer the question, so teaching them that is probably not the best strategy.

rasen58,

The way you did it is right. Converting to molar mass of M is a simple matter of looking at the equation you use to calculate number of moles or molar mass and substituting in the numbers where appropriate (you have number of moles and mass of M). Once you have that, all that's left is to correlate the molar mass of M with one of the answers available to you and see which one it matches.

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Rasen58, do you feel confused by mine post? I am really sorry for that..

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Sensei, it actually helped a lot! It helped me to figure out the way to do it without molecules. Thanks a lot!

hypervalent_iodine, I'm still not sure what to do next. How would I find the molar mass from 1 g and .0367 moles?

Edited by rasen58

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Atomic masses

Li :6.94

Fe : 55.85

Al : 26.98

B : 10.81

Au : 196.97

What does 0.0367 times each one of these come to?

Notice anything?

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Sensei, it actually helped a lot! It helped me to figure out the way to do it without molecules. Thanks a lot!

Practice is better than theory I have produced plenty of Al2O3 . 3H2O

At the beginning of production they can look like long ultra thin fibers floating in water.

You can see couple photos here:

It needs to be heated (or leaved until water vaporize) to have pure Al2O3

One is floating on water (for year or so), because bubbles have hydrogen and oxygen gases.

Edited by Sensei

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Sensei, it actually helped a lot! It helped me to figure out the way to do it without molecules. Thanks a lot!

hypervalent_iodine, I'm still not sure what to do next. How would I find the molar mass from 1 g and .0367 moles?

What does the unit g/mol mean? ##### Share on other sites

What does the unit g/mol mean? rasen58, ^ this.

Dimensional analysis is an incredibly useful tool if you don't know a formula straight away. Otherwise, you should know that number of moles = mass / molar mass. You can rearrange that equation to give you molar mass.

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I got it now. Thanks everyone!

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