# Calculate molecular mass from chemical formula C9H15N4O8P (e.g.) with rdkit?

Can I calculate the molecular mass from a formula like this C9H15N4O8P in rdkit? I think rdkit.Chem.Descriptors.MolWt needs a molecule object. Though, to generate that I would need e.g. a SMILES string. Is there any other python module that can do the trick?

As you mentioned, rdKit does not accept a chemical formula as input. The main reason for this are probably isomers. You could however use rdKit to initiate all the atoms, get their mass and add them together.

import re
from rdkit import Chem

def get_mass(formula):

parts = re.findall("[A-Z][a-z]?|[0-9]+", formula)
mass = 0

for index in range(len(parts)):
if parts[index].isnumeric():
continue

atom = Chem.Atom(parts[index])
multiplier = int(parts[index + 1]) if len(parts) > index + 1 and parts[index + 1].isnumeric() else 1
mass += atom.GetMass() * multiplier

return mass


Then...

>>> get_mass("C9H15N4O8P")
338.21299999999997