Is there any difference between oxalic acid and oxalic acid dihydrate? Is there difference in their molar mass? Is the molar mass of any substance always g/mol no matter the substance?

Answer 1

Oxalic acid is H2C2O4.
Oxalic acid dihydrate is H2C2O4.2H2O
Of course there is a difference in molar mass. The dihydrate has an additional 36 g/mol because of the two H2O moleculeds.
Molar mass can be any unit you want it to be but if you use the numbers in the periodic table you get g/mol.