Simple program to calculate molar mass of molecules with formulae without braces

molar_mass.py

ELEMENT_TABLE = {'C':12.011, 'O':15.999, 'H':1.008, 'N':14.007, 'Cl':35.45, 'Br':79.904, 'K':39.098, 'S':32.06}
def molar_mass(name, elements=ELEMENT_TABLE):
    buf = ''
    digit = ''
    molar_mass = 0.0 
    for i in range(len(name)):
        c = name[i]
        nxt = ''
        if not (i == len(name) - 1): 
            nxt = name[i + 1]

        if nxt.isupper() or nxt == '': 
            if c.isupper():
                buf += c
                molar_mass += elements[buf]
                buf = ''
            elif c.islower():
                buf += c
                molar_mass += elements[buf]
                buf = ''
            elif c.isdigit():
                digit += c
                molar_mass += elements[buf] * int(digit)
                buf = ''
                digit = ''
        elif nxt.islower():
            if c.isupper():
                buf += c
            elif c.islower():
                print("invalid expression : consecutive lower case letters.")
                break
            elif c.isdigit():
                print("invalid expression : digit followed by lower case letter.")
                break
        elif nxt.isdigit():
            if c.isupper():
                buf += c
            elif c.islower():
                buf += c
            elif c.isdigit():
                digit += c
    return molar_mass