The percent mass of potassium in K3Fe(CN)6 is 35.62%. The molar mass (Mr) of K3Fe(CN)6 is the sum of atomic masses (Ar) of its elements. Ar(K) = 39.1 g/mol, Ar(Fe) = 55.84 g/mol; Ar(C) = 12.01 g/mol; Ar(N) = 14.01 g/mol. Mr(K3Fe(CN)6) = 3Ar(K) + Ar(Fe) + 6Ar(C) + 6Ar(N) = 3 * 39.1 + 55.85 + 6 * 12.01 + 6 * 14.01 = 117.3 + 55.85 + 72.06 + 84.06 = 329.27 g/mol. There are 3 atoms of K, so total mass of K in K3Fe(CN)6 is: 3Ar(K) = 3 * 39.1 = 117.3 g/mol. In percents, this is: 329.27 : 100% = 117.3 : x. x = 100% * 117.3 : 329.27 = 35.62%
