Mr(CaCr2O7) = Mr = 7Ar(O) + Ar(Ca) + 2Ar(Cr) = 7 * 16 + 1 * 40 + 2 * 52 = 256
w(O) = 7Ar(O) / Mr = 112 / 256 = 0.4375 = 43.75%
w(Ca) = Ar(Ca) / Mr = 40 / 256 = 0.15625 = 15.625%
w(Cr) = 2Ar(Cr) / Mr = 104 / 256 = 0.40625 = 40.625%
