def Fourier_mass(t, L, D):
    """Calculates mass transfer Fourier number or `Fo` for a specified time
    `t`, characteristic length `L`, and diffusion coefficient `D`.

    .. math::
        Fo = \\frac{D t}{L^2}

    Parameters
    ----------
    t : float
        time [s]
    L : float
        Characteristic length [m]
    D : float
        Diffusivity of a species, [m^2/s]

    Returns
    -------
    Fo : float
        Fourier number (mass) []

    Notes
    -----
    .. math::
        Fo = \\frac{\\text{Diffusive transport rate}}{\\text{Storage rate}}

    Examples
    --------
    >>> Fourier_mass(t=1.5, L=2, D=1E-9)
    3.7500000000000005e-10

    References
    ----------
    .. [1] Green, Don, and Robert Perry. Perry's Chemical Engineers' Handbook,
       Eighth Edition. McGraw-Hill Professional, 2007.
    """
    return t * D / L ** 2
