Molecular Mass and Formula Mass

The Molecular Mass - is the sum of atomic masses of all the atoms present in one molecule of a substance. For example molecular mass of CO is 44 a.m.u and H O is 18 a.m.u .

Calculate the molecular mass of HNO3

Solution Atomic mass of H = 1 a.m.u Atomic mass of N = 14 a.m.u Atomic mass of O = 16 a.m.u Molecular mass = 1(At. Mass of H) +1(At. Mass of N) +3(At. Mass of O) = 1 + 14 +3(16) = 1 + 14 + 48 = 63 a.m.u

Formula Mass - The ionic compounds which form three dimensional solid crystals are represented by their formula units

Calculate the Formula mass of $Al_2(SO_3)_4$

Solution

Atomic mass of Al = 26.98 a.m.u

Atomic mass of S = 32 a.m.u

Atomic mass of O = 16 a.m.u

Formula unit = $Al_2(SO_3)_4$

Formula mass of $Al_2(SO_3)_4$ = 2(26.98) + 3(32) +12(16) 2 4 3

= 53.96 + 69 + 192

= 342.14 a.m.u