def henry(molar_weight, solubility, vapor_pressure):
    """
    Return the henry's law constant []. if < 3e-7, can be neglected
    source : (“CHEMMAP technical User’s manual 6.10,” 2014)

    params
    ------
    molar_weight[kg/mol] (in the ref, the dimension is not defined)
    solubility [kg/m³]
    vapor_pressure [Pa]
    """
    P = vapor_pressure/101325
    s = solubility * 1000
    mw = molar_weight / 1000

    return P / (s/mw)