1. Hidroxid de calciu - Ca(OH)₂
[tex]M=A_{Ca}+2A_O+2A_H=40+(2{\cdot}16)+(2{\cdot}1)=74\ g/mol\\\\\%Ca=\frac{40{\cdot}100}{74}=54.05\%\\\\\%O=\frac{(2{\cdot}16){\cdot}100}{74}=43.24\%\\\\\%H=\frac{(2{\cdot}1){\cdot}100}{74}=2.70\%[/tex]
2. Azotat de magneziu - Mg(NO₃)₂
[tex]M=A_{Mg}+2A_N+6A_O=24+(2{\cdot}14)+(6{\cdot}16)=148\ g/mol\\\\\%Mg=\frac{24{\cdot}100}{148}=16.21\%\\\\\%N=\frac{(2{\cdot}14){\cdot}100}{148}=18.91\%\\\\\%O=\frac{(6{\cdot}16){\cdot}100)}{148}=64.86\%[/tex]
3. Carbonat de potasiu - K₂CO₃
[tex]M=2A_K+A_C+3A_O=(2{\cdot}39)+12+(3{\cdot}16)=138\ g/mol\\\\\%K=\frac{(2{\cdot}39){\cdot}100}{138}=56.52\%\\\\\%C=\frac{12{\cdot}100}{138}=8.69\%\\\\\%O=\frac{(3{\cdot}16){\cdot}100}{138}=34.78\%[/tex]
4. Oxid de aluminiu - Al₂O₃
[tex]M=2A_{Al}+3A_O=(2{\cdot}27)+(3{\cdot}16)=102\ g/mol\\\\\%Al=\frac{(2{\cdot}27){\cdot}100}{102}=52.94\%\\\\\%O=\frac{(3{\cdot}16){\cdot}100}{102}=47.05\%[/tex]
5. Hidroxid de amoniu - NH₄OH
[tex]M=A_N+4A_H+A_O+A_H=14+(4{\cdot}1)+16+1=35\ g/mol\\\\\%N=\frac{14{\cdot}100}{35}=40\%\\\\\%H=\frac{(5{\cdot}1){\cdot}100}{35}=14.28\%\\\\\%O=\frac{16{\cdot}100}{35}=45.71\%[/tex]
OBS.: Dacă se adună toate procentele aferente fiecărui element din compoziția substanței, procentul final trebuie să fie de 100% (din cauza aproximărilor, procentul va fi de 99.98% sau de 99.99%, deci cu o eroare f. mică).
