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percentage composition is the mass ratio of the atoms in the molecule,

while emperical formula is the mole ratio

This is very important if your new to chemistry. Basic definition

For example, for the substance C2H2

The emperical formula is CH, which shows that the carbon mole number : hydrogen mole number is 1:1

ie. 2:2 in c2h2 simplifies to 1:1

But, the percentage composition for this will be

Percentage of C = 24/(24+2) = 92.3%

While percentage of h = 2/(24+2) = 7.69%

C: H here is 92.3 : 7.69

this is the ratio of the mass

So conclusion , percentage composition is ratio of mass while empirical formula's the ratio of mole.

the percentage composition is the amount of substance (%) in a molecule or compound. eg. in HCl there maybe a 35% of Cl nd 65% H. The empirical formula is that formula that expresses the actual compound or molecule eg. HCl or 02