Chemistry AVERAGE WEIGHT

Average Atomic Weight :

For elements, which have atoms with different relative masses (isotopes) the atomic mass is taken as weighted mean of the atomic masses.

Average atomic weight= `sum %` of Isotope `xx` Atomic Wt. of Isotope

For example, chlorine contains two isotopes of atomic masses `35` amu and `37` amu. The relative abundance of these two is in the ratio of `3:1`. Thus the atomic mass of chlorine is the average of different relative masses. Therefore atomic mass of chlorine is equal to `(35 xx 3 + 37xx 1)/4 = 35.5` amu.

Average Molecular Weight :

For homogenous mixture of several substances having number of moles, `n_i` & molecular mass `M_((i))` for `i^(th)` species, the average molecular weight is given as

Average Molecular wt. `=(n_1M_1+n_2M_2+...........)/(n_1+n_2+.............)`

`=text(Total weight)/text(Total no of Moles)=(sum n_i M_i)/(sum n_i)`

Minimum Molecular Weight:

It is the molecular weight of a compound shown by presence of minimum number of atoms [i.e. for monomer = 1, Dimer = 2, Trimer = 3, Tetramer = 4 ]

e.g. Insulin contains `3.4%` of Sulphur. Find its Minimum Molecular Weight.

`100` g insulin contains `3.4` g `S` (sulphur).

Assuming it to be monomer [as nothing specified] one sulphur atom has `32` amu (atomic mass) , We can write , `3.4` g `S` is contained by `100` g insulin
`32` g `S` is contained by = `(100/3.4)xx32` = Minimum Molecular Weight