How Do You Find the Molecular Formula for a Compound with a Given Molar Mass?

[PrudensOptimus]

Math Rules Chem:)

$$NO_3$$

What is the molecular formula of this compound if molar mass is 324.06.

Math begins:

$$aN + bO = 324.06$$

$$N=14, O = 16$$

$$14a + 16b = 324.06$$

$$N = O/3, O = 3N, a = b/3, b = 3a$$

$$a = 162.03/32 =~ 5, b =~ 15.$$

$$N_{5}O_{15}$$

 

[BluE]

If N=14 and and N=O/3, then 14=O/3 so O=14/3=4.66666... but you say O=16. I think I'm confused.

 

[PrudensOptimus]

Originally posted by BluE
If N=14 and and N=O/3, then 14=O/3 so O=14/3=4.66666... but you say O=16. I think I'm confused.

How old are you?

 

[sven222]

Is there a point to this thread?

 

[BluE]

How old are you?

What, did I say something wrong?


Oh. Now I see. 14 and 16 are the amu of them. I didn't look at it.

 

[ShawnD]

Originally posted by PrudensOptimus
$$NO_3$$

What is the molecular formula of this compound if molar mass is 324.06.

Math begins:

$$aN + bO = 324.06$$

$$N=14, O = 16$$

$$14a + 16b = 324.06$$

$$N = O/3, O = 3N, a = b/3, b = 3a$$

$$a = 162.03/32 =~ 5, b =~ 15.$$

$$N_{5}O_{15}$$

Your work is wrong. The question has no answer. First it TELLS you the formula then it asks for a formula OF THE SAME COMPOUND but with a different mass.
Anybody who knows anything about chemistry will tell you that the same compound will have the same formula. You can't just multiply each of the subscripts by a common factor to get a new formula for "the same compound", it doesn't work like that. OH and H2O2 are NOT the same. They don't have the same name, properties, or even charge. Let's look at another formula; C3H8. If we multiply the subscripts by 2 we get C6H16. The problem is that C6H16 doesn't even exist; it has too many hydrogens.

You can't just multiply the subscripts by a common factor like that.

 

[FZ+]

ShawnD: I think you need to look into the context of this particular question. The first line is the empirical formulae, which is essentially just the ratios of atoms in each molecule of the compound. The answer PO derived is correct.

Though IMHO, he dragged it out much more than he should have. The simplest way is just to calculate molar mass for the empirical formula, divide this from your given real mass, and multiple each subscript by the result of the above.

 

[PrudensOptimus]

Originally posted by FZ+
ShawnD: I think you need to look into the context of this particular question. The first line is the empirical formulae, which is essentially just the ratios of atoms in each molecule of the compound. The answer PO derived is correct.

Though IMHO, he dragged it out much more than he should have. The simplest way is just to calculate molar mass for the empirical formula, divide this from your given real mass, and multiple each subscript by the result of the above.

I should write a research paper on this :) Using real math to solve chemistry.

 

[Monique]

Originally posted by PrudensOptimus
I should write a research paper on this :) Using real math to solve chemistry.

Too bad your math does not take into the equation the chemical relevancy of the answer.

As Shawn said, C₃H₈ exists, but C₆H₁₆ (everything times 2) does not.

 

[PrudensOptimus]

Originally posted by Monique
Too bad your math does not take into the equation the chemical relevancy of the answer.

As Shawn said, C₃H₈ exists, but C₆H₁₆ (everything times 2) does not.

I don't know what you are talking about because I'm still teaching myself chemistry... and probably some advance stuff u were mentioning.

Anyways, like every other thing, there was a pioneer. :) Be glad you replied to this thread today, because ...

 

[Monique]

There already is a formula for the kind of compound I mentioned (alkanes):

H = 2x C + 2

So it is C₃H₈ and C₆H₁₄.

 

[PrudensOptimus]

Originally posted by Monique
There already is a formula for the kind of compound I mentioned (alkanes):

H = 2x C + 2

So it is C₃H₈ and C₆H₁₄.

What does that have to do with NO?

 

[Khartoum]

they're just proving that you can't just have hydroxide and h2o2 equal in your equations