molecular formula question

[binglordi]

sorry if this is so stupid, i dont start chem until next year.

 

but how do scientists determine the molecular formula of chemicals?

 

for example, how do scientists know that water has two hydrogen atoms and one oxygen?

 

or lets pretend glucose is an unknown substance, how would a scientist find out it has 6 carbon 12 hydrogen 6 oxygen?

[hypertilly]

This is a good question.

I don't know the answer either and I'd like to know too.

 

Could it be some kind of microscope?

[Gypsy Cake]

I reckon it's spectroscopy. I know that spectroscopy shows what bonds are present. Though I haven't done it in detail yet so I'm not sure if everything can be worked out like that.

[thechronic]

I've often wondered this myself, but I don't know.

[hypertilly]

I consulted a friend, and heres what he said.

To answer your question about how we determine the chemical formula for a compound, here's how it works. There are a variety of techniques we can use to dermine what elements are present in a particular chemical. These techniques include: gas-chromotography/mass-spectrometery(GCMS), nuclear magnetic resonance (NMR), infrared spectrosocopy (IR) and a few other chromatographic methods. These techniques can identify what elements are present and what percentage of the compound it makes up. I don't know how much detail you are looking for here but I can explain this further if you would like. I would recommend that you go to google and look up the information on GCMS, NMR and IR and that will help you understand how each one of these work.

[binglordi]

thank you & your friend.

 

i was a little more interested in how this was done before all these fancy machines and techniques were discovered. the answer appears to be combustion analysis?

 

i google'd around for hours before coming across what i think is the correct broad subject term, elemental analysis?

[hypertilly]

Did you see this site?

I thought the diagram was pretty handy.

 

http://www.preparatorychemistry.com/bishop_combustion_analysis.htm

[woelen]

Before the use of advanced electronic equipment, determination of empiric formula's of compounds was a real art.

 

Determining the ratio of C, H, O and N in organic compounds was done by completely burning the compound, such that only gaseous compounds remained. The burning was done VERY precisely, taking into account all oxygen which was added, and measuring the weight of all combustion products (water was captured first with P4O10 or H2SO4, then CO2 was captured with NaOH-solution, the remains were nitrogen, or simply 'other'). With some basic math, they could determine the ratio of C, H, O and 'other'. A compound like C6H12O6 would end up like CH2O, benzene was CH.

 

The next step of determining the precise molecular net-formula was even trickier. It depended on the nature of the compound. It could be dissolved, and the drop in freezing temperature of the solution was a measure for the concentration of molecules. E.g. 1 gram of C6H12O6 gives a 6 times as low a concentration (and lower drop of freezing point) as 1 gram of CH2O. This kind of experiment was very tedious and required a lot of patience.

 

Another way of determining net molecular weights was by evaporating the compound (if possible) and measuring the density of the gas at a known temperature. In this way it was discovered that methane really was CH4 and not C2H8, and chlorine was Cl2 and not Cl, or Cl3.