can you determine the molecular formula of a substance from its percents composition?

Answer 1

sometimes, if it's a simple molecule - but usually not

Answer 2

Sometimes, when the compound is simple enough, you may assume the molecular formula which correspond to the percent composition, aided by the sample origin, visual inspection and maybe a couple of simple test. Take for example, a specimen made of around 66%copper and 33%sulfur. If it has a beatiful blue colour, it might be covellite (CuS). However, a physical mixture of fine grounded pure copper and suflur powder will yield the same composition, but the appearance would be very obvious to distinguish under a magnifying lens. Moreover, shaking it might cause seggregation.

The above example might be stupid, but , when the nature of a compound is not obvious even if the elemental composition is known, one may need a more sophisticated technique, such as;

X-ray diffraction
IR spectroscopy with Fourier Transforms
Nuclear Magnetic Resonance
ESCA
SEM-EDS
Optical petrography/microscopy

One of the great things my eyes have seen in recent times is the amazing analytical techniques available nowadays. Normally, a combination of techniques gives the best results. It will depend a lot on the dexterity of the researcher and the knowledge of the sample nature.

Answer 3

YES!

The Molecular Formula is just a ratio of atoms of each element in a molecule. If you know the percent composition, you can calculate the number of atoms present for each element. Then you'll know their ratio and the molecular formula.

I'm assuming you know the actual weight along with the percent composition, measured in grams or whatever.

Answer 4

You can determine the "empirical formula" knowing the percentages of each element in the molecule. However, you cannot determine its "molecular formula". Consider the following-

C2H2 acetylene
C6H6 benzene

Both have the same percentage composition.

Answer 5

NO. Think about all the biologically interesting macromolecules. It does not help to know that it has 512 carbons and xx nitrogen and yy oxygen and zzz hydrogen and m sulfur!

Edit: Hm, I didn't read the question. The answer is still no, though. It does not help to know that it's 20% carbon and 40% hydrogen and ... if your molecule isn't trivial. There are a huge number of combinations that are used in organic chemistry -- the more carbons you have, the less useful such information is.

Answer 6

if u know percent composition yes it can be calculated

it is a very common question in general chemistry lectures as find the emprical formula ? :)

Answer 7

yes.