Here's how to calculate its molecular mass:

* Na (Sodium): 2 atoms * 22.99 g/mol = 45.98 g/mol

* C (Carbon): 1 atom * 12.01 g/mol = 12.01 g/mol

* O (Oxygen): 3 atoms * 16.00 g/mol = 48.00 g/mol

* H (Hydrogen): 20 atoms * 1.01 g/mol = 20.20 g/mol

Total Molecular Mass = 45.98 + 12.01 + 48.00 + 20.20 = 126.19 g/mol

Therefore, the molecular mass of washing soda (sodium carbonate decahydrate) is 126.19 g/mol.