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Chemistry Forums for Students => Undergraduate General Chemistry Forum => Topic started by: Edgar9760 on October 10, 2011, 04:32:15 AM

Title: How do I figure out empirical formulas when I'm given mol of certain atoms?
Post by: Edgar9760 on October 10, 2011, 04:32:15 AM

The problem given is:
Determine the empirical formulas of the following compounds:
(a) 0.039 mol of iron atoms combined with 0.052 mol of oxygen atoms

(b) 0.903 g of phosphorous combined with 6.99 g of bromine

(c) A hydrocarbon with 79.9 mass % carbon

I don't know where to start.

Title: Re: How do I figure out empirical formulas when I'm given mol of certain atoms?
Post by: Edgar9760 on October 10, 2011, 04:43:36 AM

I started (a) and all I could think of was converting the moles given moles to grams getting the following:
2.18g of iron and 0.832g of oxygen.

Title: Re: How do I figure out empirical formulas when I'm given mol of certain atoms?
Post by: Borek on October 10, 2011, 04:55:02 AM

So you already went in the wrong direction. Empirical formula is about ratio of number of atoms in the molecule - that is much closer to moles than to masses.

Title: Re: How do I figure out empirical formulas when I'm given mol of certain atoms?
Post by: Edgar9760 on October 10, 2011, 05:01:52 AM

Ok I tried once more and the answer I got was Fe3O4. The numbers being subscripts. Is that correct?

Title: Re: How do I figure out empirical formulas when I'm given mol of certain atoms?
Post by: Borek on October 10, 2011, 06:02:04 AM

Fe₃O₄ it is.

Title: Re: How do I figure out empirical formulas when I'm given mol of certain atoms?
Post by: Edgar9760 on October 10, 2011, 08:07:06 AM

Thank you. As for part (c), I'm not sure which hydrocarbon the problem is referring to. How do I find that out?

Title: Re: How do I figure out empirical formulas when I'm given mol of certain atoms?
Post by: fledarmus on October 10, 2011, 08:29:33 AM

What is the definition of a hydrocarbon?

Title: Re: How do I figure out empirical formulas when I'm given mol of certain atoms?
Post by: Edgar9760 on October 10, 2011, 06:41:58 PM

A compound of hydrogen and carbon, such as any of those that are the chief components of petroleum and natural gas.

Title: Re: How do I figure out empirical formulas when I'm given mol of certain atoms?
Post by: fledarmus on October 10, 2011, 09:16:34 PM

Good - and if it contains only hydrogen and carbon, and it is 79.9% carbon, what would the molecular formula be?