Molar mass

import re
# elements and their mass
elements = {
  'C' : 12,
  'H' : 1,
  'N' : 14,
  'O' : 16,
  'S' : 32,
  'Cl' : 35.5,
  'P' : 31
}

def molar_mass(formula):
    """ Returns a molar mass of compound.
        Uses single argument - formula of compound
    """
    s = 0
    for i in elements.keys():
        f = re.findall(i+'[0-9]*',formula)
        if f:
            tmp = f[0]
            num = re.findall('[0-9]+',tmp)
            if num == []:
                s += elements[tmp]
            else:
                s += elements[i] * int(num[0])
    return s