def standard(target, mol_weight='pore.molecular_weight', molar_density=
    'pore.molar_density'):
    """
    Calculates the mass density from the molecular weight and molar density

    Parameters
    ----------
    target : OpenPNM Object
        The object for which these values are being calculated.  This
        controls the length of the calculated array, and also provides
        access to other necessary thermofluid properties.

    mol_weight : string
        The dictionary key containing the molecular weight values (kg/mol)

    molar_density : string
        The dictionary key containing the molar density values (mol/m3)

    Returns
    -------
    value : NumPy ndarray
        Array containing density values (kg/m3)

    """
    MW = target[mol_weight]
    rho = target[molar_density]
    value = rho * MW
    return value
