$\rm d.M_{RSO_4}=5,825M_{Ca}$

`=>` $\rm M_{RSO_4}=5,825.40=233(amu)$

`=>` $\rm M_R+M_S+4.M_O=233 \to M_R+32+4.16=233$

`=>` $\rm M_R=137(amu)$

vậy $\rm R: barium ( Ba)$

$\rm e.M_{Fe_2(RO_4)_3}=12,5.M_{O_2}$

`=>` $\rm M_{Fe_2(RO_4)_3}=12,5.16.2=400(amu)$

`=>` $\rm 2M_{Fe}+3M_R+3.4M_O=400 \to 2.56+3M_R+3.4.16=400$

`=>` $\rm M_R=32(amu)$

vậy $\rm R: sulfur (S)$