def y_aver_bot(yw_mol, ypf_mol):
    """
    Calculates the average mol concentration  at bottom of column.
    Parameters
    ----------
    yw_mol : float
        The mol concentration  of waste, [kmol/kmol]
    ypf_mol : float
        The mol concentration of point of feed, [kmol/kmol]  
    Returns
    -------
    y_aver_top : float
        The average mol concentration at top of column, [kmol/kmol]
    References
    ----------
    Дытнерский, стр.230, формула 6.8
    """            
    return (yw_mol + ypf_mol) / 2