def ws_to_zs(ws, MWs):
tot = sum(w/MW for w, MW in zip(ws, MWs))<EOL>zs = [w/MW/tot for w, MW in zip(ws, MWs)]<EOL>return zs<EOL>
r'''Converts a list of mass fractions to mole fractions. Requires molecular
    weights for all species.

    .. math::
        z_i = \frac{\frac{w_i}{MW_i}}{\sum_i \frac{w_i}{MW_i}}

    Parameters
    ----------
    ws : iterable
        Mass fractions [-]
    MWs : iterable
        Molecular weights [g/mol]

    Returns
    -------
    zs : iterable
        Mole fractions [-]

    Notes
    -----
    Does not check that the sums add to one. Does not check that inputs are of
    the same length.

    Examples
    --------
    >>> ws_to_zs([0.3333333333333333, 0.6666666666666666], [10, 20])
    [0.5, 0.5]