The molar mass of dibasic sodium phosphate or (Na2HPO4) is 141.96 grams/mole.The molar masses of the individual elements are:Na= 22.99 grams H= 1.008 grams P= 30.97 grams O= 16.00 grams Now just divide the mass of each element present in the compound by the molar mass and multiply by 100 for the percent composition.