A molar mass calculator

You will build a small calculator: a function that finds the molar mass of a formula written as a list of (symbol, number of atoms), a function for the percentage of one element, and a search for the heaviest of five substances.

The idea. Molar mass = the sum of (number of atoms × atomic mass). The share of an element = its mass in the formula ÷ the molar mass × $100$.

Your tasks.

1. In molar_mass, add n * atomic[symbol] for every part of the formula.
2. In percent, return the percentage of element in the formula.
3. Find heaviest, the name of the substance with the largest molar mass.