Washing soda is sodium carbonate, Na2CO3.

Molar mass $=$ the sum of atomic mass times count for every element. In Python: sum(atomic[el] * n for el, n in counts.items()). A formula is read into symbols and counts with re.findall(r"([A-Z][a-z]?)(\d*)", formula), after brackets are opened. Mass percent $= \dfrac{\text{atomic mass} \times \text{count}}{M} \times 100$.