Gram formula mass is the mass of 1 mol of substance.
the given compound is Fe(NO₃)₃
the molar mass of Fe(NO₃)₃ can be calculated taking the sum of the products of the individual elements by the number of atoms of the element making up the compound.
molar mass of Fe(NO₃)₃ = molar mass of Fe + 3 (molar mass of N + (3 x molar mass of O))
molar mass of Fe - 56, N - 14 and O - 16 g/mol
substituting the values in the above formula
molar mass of Fe(NO₃)₃ = 56 + 3 (14 + (3 x 16) )
molar mass of Fe(NO₃)₃ = 56 + 3 (14 + 48) = 56 + 3(62) = 56 + 186 = 242
therefore molar mass of Fe(NO₃)₃ is 4)242 g/mol