def sitearea_fromqcm(M, mpc, nmol=1):
    """Average area of a surface site

    Calculate the average area of a surface site from qcm data

    Parameters
    ----------

    M : float
        Molecular mass in atomic mass units
    mpc : float
        Mass per cycle in  ng/cm2
    nmol : int, optional (default 1)
        Number of precursor molecules per unit formula of the solid

    Returns
    -------
    float
        Average area of a surface site in sq. meters

    """

    return M/(mpc*1e-5*6.022e23*nmol)