K3PO4
M(K3PO4) = 39*3+31+16*4 = 212 g/mol
W(K) = 39*3 / 212 * 100% = 55.19%
W(P) = 31 / 212 * 100% = 14.62%
W(O) = 16*4 / 212*100% = 30.19%
FeSO4
M(FeSO4) = 56+32+64 = 152 g/mol
W(Fe) = 56 / 152*100% = 36.84%
W(S) = 32/ 152 * 100% = 21.05%
W(O) = 16*4 / 152*100% = 42.11%