Where do the percents in empirical formula questions come from?

A typical empirical formula problem reads something like this:

Determine the empirical formula for a compound that contains 35.98% aluminum and 64.02% sulfur. source

How would those percentages be determined in real life? Spectroscopy?

A percent composition problem would obviously yield percentages, but those start with compound formulas--there'd be no need to determine the empirical formula from the percentages.

• I believe its through chemical processes like combustion(the sulphur would form $\ce{SO2}$, which you can then capture and quantitatively estimate), reactions with acids, alkalis, halogens, titrations(if it's an acid) etc. Feb 12, 2020 at 16:23
• It is impossible to make any general statements about how the atomic percentages would be determined. There are a zillion different kinds of samples and a zillion different analytical techniques.
– MaxW
Feb 12, 2020 at 17:35

How would those percentages be determined in real life? Spectroscopy?

You are right, this analysis comes in the domain of analytical chemistry. The fundamental requirement for formula determination is that your sample must be extremely pure.

There is a whole field. So there are two broad classes of approaches. One is called classical analysis where one would dissolve the sample in an "appropriate" acid or base mixture, and this is followed by target element specific titration or precipitation. This technique results in very reliable data but it is very time consuming

Today most people would use an instrument called the inductively coupled plasma optical emission spectroscopy (ICP-OES), you "srpay" the dissolve sample into a state of matter (plasma) which is as hot as the surface of the Sun. Under those those conditions, compounds break down into their constituent atoms and emit characteristic wavelengths. For example, if we are interested in knowing how much aluminium is there, we would construct a calibration curve and interpolate the amount of Al in your unknown sample. The same thing (although less effectively) can be done for sulfur.

Sometimes you need both classical or instrumental analysis.

These percentage are experimental. Suppose you mix 36 g aluminium powder and 64 g sulfur powder, and you heat the mixture. A exothermic reaction will occur. As a final result, a pure compound is produced, which is aluminium sulfide. With any other proportion, the same aluminium sulfide is formed, but some aluminium or some sulfur remains unchanged at the end of the reaction.

• Thank you for the attempt, but this answer does not address the method of analysis. Feb 18, 2020 at 19:01