def MW(CASRN):
return pubchem_db.search_CAS(CASRN).MW<EOL>
Given a CASRN in the database, obtain the Molecular weight of the
    compound, if it is in the database.

    Parameters
    ----------
    CASRN : string
        Valid CAS number in PubChem database

    Returns
    -------
    MolecularWeight : float

    Notes
    -----
    CASRN must be an indexing key in the pubchem database. No MW Calculation is
    performed; nor are any historical isotopic corrections applied.

    Examples
    --------
    >>> MW('7732-18-5')
    18.01528

    References
    ----------
    .. [1] Pubchem.