def xf_mol(xf_mass, M_lc, M_hc):
    """
    Calculates the mol composition of feed.
    Parameters
    ----------
    xf_mass : float
        The mass concentration low component of feed, [kg/kg]
    M_lc : float
        The molar mass of low-boilling component, [kg/kmol]
    M_hc : float
        The molar mass of high-boilling component, [kg/kmol]
    Returns
    -------
    xf_mol : float
        The mol concentration low component of feed, [kmol/kmol]
    References
    ----------
    Романков, стр.283, табл. 6.2. 
    """      
    return (xf_mass * M_hc) / ((xf_mass * M_hc) + (M_lc - M_lc * xf_mass))