Mr(SiO2) = 28 + 16*2 = 60
W(O) = Ar(O) * n / Mr(SiO2) *100% = 16 * 2 / 60 *100% = 53,3%
Mr(Al2O3) = 27*2 + 16 *3 = 102
W(O) = Ar(O) * n / Mr(Al2O3) *100% = 16*3 / 102 *100% = 47%
Mr(H2CO3) = 1*2 + 12 + 16*3 = 62
W(O) = Ar(O) * n / Mr(H2CO3) *100% = 16*3 / 62 *100% = 77,4%