An unknown metal sulfate is found to be 72.07% so4 2- by mass. assuming that the charge on the metal cation is +3, determine the identity of the cation.

Lets name the unknown metal as M. Cation would be M³⁺.
the molecular formula of the compound is M₂(SO₄)₃
the mass of one mole - (molar mass of M x2 + 3 x molar mass of SO₄²⁻)
                                   = 2M + 96 x 3
                                   = 2M + 288
In 1 mol if there's 72.07% of sulphate , 
then 72.07 % corresponds to 288 g
               1 % is then      - 288/72.07 
       100 %  of the compound - 288/72.07 x 100 
 molar mass of the compound - 399.6 g/mol
mass of 2M - 399.6 - 288 = 111.6 g
molar mass of M - 111.6 /2 = 55.8 g/mol
the element with molar mass of 55.8 is Fe.
Unknown metal is iron(III) , Fe³⁺

The mass has escaped into a gas which you can not see so the law stays true.