+)
`P_2O_5`: `31.2 + 16.5 = 142` `(đvC)`
`->` `%P = \frac{31.2}{142} xx 100% = 43,66%`
`->` `%O = 100% - 43,66% = 56,34%`
+)
`Al_2(SO_4)_3`: `27.2 + 32.3 + 16.12 = 342` `(đvC)`
`->` `%Al = \frac{27.2}{342} xx 100% = 15,78%`
`->` `%S = \frac{32.3}{342} = 28,07%`
`->` `%O = 100% - 15,78% - 28,07% = 56,15%`