H2SO4
H: 1 x 2 = 2
S: 32 x 1 = 32
O: 16 x 4 = 64
98amu
H: 2/98 x 100 = 2%
S: 32/98 x 100 = 33%
O: 64/98 x 100 = 65%
MGNO3 (I think you meant Mg(NO3)2 aka Magnesium Nitrate. So this might be wrong because I did the version you sent instead of what I thought you might mean.)
Mg: 24 x 1 = 24
N: 14 x 1 = 14
O: 16 x 3 = 48
86amu
Mg: 24/86 x 100 = 28%
N: 14/86 x 100 = 16%
O: 48/86 x 100 = 56%
