you need to find the whole molar mass of the compound using the periodic table to add the values.
Na2CO3= (2 x 23.0) + 12.0 + (3 x 16.0)= 106 g/mol
H2O= 10 x [ (2 x 1.01 ) + (16.0) ]= 180.2 g/mol
the total molar mass is 106 + 180.2 = 286.2 g/mol
the percentage of water you can find by doing "parts over the whole"
H2O%= 180.2 / 286.2 X 100= 63.0%
