а) $M_r$ ($C{O}_2$) = 12 + 2 * 16 = 44
$w$(C) = $\frac{12}{44}$ * 100% = 27%
$w$(O) = $\frac{2\ast <!--\; \ast \; -->16}{44}$ * 100% = 73%
б) $M_r$ (CaS) = 40 + 32 = 72
$w$(Ca) = $\frac{40}{72}$ * 100% = 55,6%
$w$(S) = 100% − 55,6% = 44,4%
в) $M_r$ ($NaN{O}_3$) = 23 + 14 + 3 * 16 = 85
$w$(Na) = $\frac{23}{85}$ * 100% = 27%
$w$(N) = $\frac{14}{85}$ * 100% = 17%
$w$(O) = 100% − 27% − 17% = 56%
г) $M_r$ ($A{l}_2{O}_3$) = 2 * 27 + 3 * 16 = 102
$w$(Al) = $\frac{2\ast <!--\; \ast \; -->27}{102}$ * 100% = 53%
$w$(O) = 100% − 53% = 47%