a) siarczek sodu - Na₂S 

Masy atomowe: 

${\text{m}}_{\text{at.Na}}=23\text{u}$  

${\text{m}}_{\text{at.S}}=32\text{u}$  

Masa cząsteczkowa: 

${\text{m}}_{\text{cz.}}\left({\text{Na}}_2\text{S}\right)=2\cdot {\text{m}}_{\text{at.Na}}+{\text{m}}_{\text{at.S}}$  

${\text{m}}_{\text{cz.}}\left({\text{Na}}_2\text{S}\right)=2\cdot 23\text{u}+32\text{u}=78\text{u}$  

Zawartość procentowa: 

$\%\text{Na}=\frac{2\cdot {\text{m}}_{\text{at.Na}}}{{\text{m}}_{\text{cz.}}\left({\text{Na}}_2\text{S}\right)}\cdot 100\%$  

$\%\text{Na}=\frac{2\cdot 23\text{u}}{78\text{u}}\cdot 100\%=59\%$  

$\%\text{S}=100\%-\%\text{Na}$  

$\%\text{S}=100\%-59\%=41\%$  

b) tlenek węgla(IV) - CO₂ 

Masy atomowe: 

${\text{m}}_{\text{at.C}}=12\text{u}$  

${\text{m}}_{\text{at.O}}=16\text{u}$  

Masa cząsteczkowa: 

${\text{m}}_{\text{cz.}}\left({\text{CO}}_2\right)={\text{m}}_{\text{at.C}}+2\cdot {\text{m}}_{\text{at.O}}$  

${\text{m}}_{\text{cz.}}\left({\text{CO}}_2\right)=12\text{u}+2\cdot 16\text{u}=44\text{u}$  

Zawartość procentowa: 

$\%\text{C}=\frac{{\text{m}}_{\text{at.C}}}{{\text{m}}_{\text{cz.}}\left({\text{CO}}_2\right)}\cdot 100\%$  

$\%\text{C}=\frac{12\text{u}}{44\text{u}}\cdot 100\%=27,3\%$   

$\%\text{O}=100\%-\%\text{C}$  

$\%\text{O}=100\%-27,27\%=72,7\%$  

c) chlorek glinu - AlCl₃ 

Masy atomowe: 

${\text{m}}_{\text{at.Al}}=27\text{u}$  

${\text{m}}_{\text{at.Cl}}=35,5\text{u}$  

Masa cząsteczkowa: 

${\text{m}}_{\text{cz.}}\left({\text{AlCl}}_3\right)={\text{m}}_{\text{at.Al}}+3\cdot {\text{m}}_{\text{at.Cl}}$  

${\text{m}}_{\text{cz.}}\left({\text{AlCl}}_3\right)=27\text{u}+3\cdot 35,5\text{u}=133,5\text{u}$  

Zawartość procentowa: 

$\%\text{Al}=\frac{{\text{m}}_{\text{at.Al}}}{{\text{m}}_{\text{cz.}}\left({\text{AlCl}}_3\right)}\cdot 100\%$  

$\%\text{Al}=\frac{27\text{u}}{133,5\text{u}}\cdot 100\%=20,2\%$  

$\%\text{Cl}=100\%-\%\text{Al}$  

$\%\text{Cl}=100\%-20,2\%=79,8\%$