Question

i cant get how to get molar mass

Answer 1

It is very easy firstly count the atoms present in the given compound.
exp.
NaCl
molar mass of Na is 22
molar mass of Cl is 34
molar mass of NaCl is 22 + 34 = 56

Answer 2

It is equal to atomic mass in gram.
O=8u( atomic mass)
O=8g(molar mass)