Cr(NO3)3
M(Cr(NO3)3)=52+(14+16*3)*3=238g/mol
W(Cr)=52/238*100=21.85
W(N)=14*3/238*100=17.65
W(O)=16*9/238*100=60.5

BaCO3
M(BaCO3)=137+12+16*3=197g/mol
W(Ba)=137/197*100=69.54
W(C)=12/197*100=6.09
W(O)=16*3/197*100=24.37

Ag2CO3
M(Ag2CO3)=108*2+12+16*3=276g/mol
W(Ag)=108*2/276*100=76.26
W(C)=12/276*100=4.35
W(O)=16*3/276*100=17.39

MgIO3
M(MgIO3)=24+127+16*3=199g/mol
W(Mg)=24/199*100=12.06
W(I)=127/199*100=63.82
W(O)=16*3/199*100=24.12

NaPO3
M(NaPO3)=23+31+16*3=102g/mol
W(Na)=23/102*100=22.55
W(P)=31/102*100=30.39
W(O)=16*3/102*100=47.06