def M_top(M_lc, x_aver_top, M_hc):
    """
    Calculates the molar mass at top of column.
    Parameters
    ----------
    x_aver_top : float
        The average mol concentration at top of column, [kmol/kmol]
    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
    -------
    M_top : float
        The molar mass at top of column, [kg/kmol]
    References
    ----------
    Дытнерский, стр. 230, формула 6.6
    """ 
    return (M_lc * x_aver_top + M_hc * (1 - x_aver_top))