The empirical formula of a compound may be defined as the formula which gives the simplest whole number ratio of atoms of the various elements present in the molecule of the compound. The empirical formula of the compound glucose `(C_5H_12O_5)` is `CH_2O` which shows that `C,H` and `O` are present in the simplest ratio of `1:2:1`

Empirical formula mass of substance is equal to the sum of atomic masses of all the atoms in the empirical formula of the substance. Molecular formula is a whole number multiple of empirical formula.

Thus Molecular formula = `text((Empirical formula))_ n` where `n = 1,2,3 ..`.

`n=text(Molecular Formula)/text(Empirical Formula)=text(Molecular mass)/text(Empirical mass)`

The percentage of the elements in the compound is determined by suitable methods and from the data collected, the empirical formula is determined by the following steps:

a.) Divide the percentage of each element by its atomic mass. This will give the relative number of moles of various elements present in the compound.
b.) Divide the quotients obtained in the above step by the smallest of them so as to get a simple ratio of moles of various elements.
c.) Multiply the figures, so obtained by a suitable integer if necessary in order to obtain a whole number ratio.
d.) Finally write down the symbols of the various elements side by side and put the above number as the subscripts to the lower right hand corner of each symbol. This will represent the empirical formula of the compound.

a.) Calculate the empirical formula as described above.
b.) Find out the empirical formula mass by adding the atomic masses of all the atoms present in the empirical formula of the compound.
c.) Divide the molecular mass (determined experimentally by some suitable method) by the empirical formula mass and find out the value of `n`.

DENSITY:
Density is of two types,

a.) Absolute Density
b.) Relative Density

For liquid and solids:

Absolute density = `text(mass)/text(volume)`

Relative density or specific gravity = `text(density of the substance)/text(density of water at 4 degree celcius)`

For Gases:

Absolute density = `text(Molar mass)/text(Molar volume) = (PM)/(RT)`

Where `P` is pressure of gas, `M` = mol. Wt. of gas , `R` is the gas constant , `T` is the temperature.

VAPOUR DENSITY :

Vapour density is defined as the density of the gas with respect to hydrogen gas at the same temperature and pressure.

Vapour density = `d_(gas)/d_(H_2) = ((PM_(gas))/(RT))/((PM_(H_2))/(RT))` `=> V. D. = M_(gas)/M_(H_2) = M_(gas)/2`

`M_(gas) = 2 (V.D.)`

Relative density can be calculated w.r. t. to other gases also.