Substantele sunt : K2O, Na2CO3, Mg3(PO4)2
Iti dau ex. la prima substanta:
-raportul atomic 2atK:1atO
-raportul de masa
mK:mO=2x39g: 16g
=39:8
-masa moleculara
M,sau miu= (2x39gK+16gO)/mol=94g/mol
-compozitia procentuala
in 94g oxid ..sunt..78gK........16gO
100g.......................x..............y
x=83% K y=17%O