Molar mass from a freezing-point drop

You will write functions that turn a fall of the freezing point into a molar mass. Then you will use them to decide which compound a chemist has dissolved.

The idea. $\Delta T_f = K_f\,m$, and the moles of solute are the molality times the mass of the solvent in kilograms.

Your tasks.

1. Complete molality(drop, kf) so that it returns the molality in mol/kg.
2. Complete molar_mass(solute, solvent, drop, kf) so that it returns the molar mass in g/mol. The masses are in grams.
3. Set guess to the name of the compound in candidates whose molar mass is closest to the measured one.