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Offline wholemilk

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Question on ratios in a chemical formula
« on: February 25, 2014, 05:46:35 PM »
Hi all,
Just picked up some chemistry books from the library and I have a question about something I read on chemical formulae to represent compounds. The example given was ammonia (NH3). This means that that ammonia is a compound made of joined up nitrogen and hydrogen atoms in the ratio of three nitrogen atoms to one hydrogen atoms.

My question therefore is, how do you determine the absolute number of atoms in a compound simply by looking at the ratio? I know that an ammonia molecule has three hydrogen and one nitrogen because I looked it up, but what's stopping the molecule from consisting of two nitrogen and six hydrogen atoms since this  arrangement would still be complying with the given ratio of 1:3?

Thank you very much in advance and apologies if this is a stupid question :p

Offline Corribus

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Re: Question on ratios in a chemical formula
« Reply #1 on: February 25, 2014, 06:32:25 PM »
Not a stupid question. Look up the distinction between the molecular formula and empirical formula.  If the molecule had two nitrogens and six hydrogens, the molecular formula would be N2H6 and the empirical formula would be NH3. Chemists almost always deal with molecular formulas, which specify the number of atoms of each element in each molecular unit. The exceptions are ionic compounds (salts) and polymers, which don't often have easily definable molecular units. In these cases the empirical formula is used, or a sort of empirical-molecular formula hybrid (CnHn, e.g., for a polymer).

Read more here:

http://en.wikipedia.org/wiki/Molecular_formula
What men are poets who can speak of Jupiter if he were like a man, but if he is an immense spinning sphere of methane and ammonia must be silent?  - Richard P. Feynman

Offline wholemilk

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Re: Question on ratios in a chemical formula
« Reply #2 on: February 25, 2014, 06:48:51 PM »
Hi!

Thanks for that, it cleared everything up!  :)

Offline acvkrs

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Re: Question on ratios in a chemical formula
« Reply #3 on: February 26, 2014, 12:12:27 PM »
good question! my ans is NH3 is stable structure(possible structure) and N2H6 is not. in your proposed N2H6, both the nitrogens in NH3 needs to be bound together with a covalent bond. if so, nitrogen gets four covalent bonds which is a violation of valency bond theory. and in addition, as nitrogen contains a lone pair of electrons, they cannot try to make a bond with another nitrogen because of the repulsions between the electrons of two N atoms. so these restrictions restricts your proposed N2H6 and so doesn,t exist.

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