A percentage composition calculator

Every compound has a fixed composition by mass. You will find the molar mass of ammonium phosphate, a fertiliser, from its formula, then the mass percent of each element, and the element with the largest share.

The idea. The molar mass adds up atomic mass times count for every element. The mass percent of an element is its atomic mass times its count, divided by the molar mass, times $100$.

Your tasks.

1. In molar_mass, return the molar mass of a formula given as a dictionary.
2. Fill percent with the mass percent of each element.
3. Set largest to the element with the largest mass percent.