You need to change them to mole
mole = mass / Molar mass
mole of Ca = 0.910 g / 40.08 g/mol = 0.0227 mol
mole of N = 0.636 g / 14.01 g/mol = 0.04539 mol
mole of O = 1.453 g / 16.00 g/mol = 0.0908 mol
Then, take the smallest number and divide for all number (in mol that I just found)
Ca = 0.0227 / 0.0227 = 1
N = 0.04539 / 0.0227 = 1.9999 (round it 2)
O = 0.0908 / 0.0227 = 4
So the empirical formula will be Ca1N2O4 ( CaN2O4 )
