(14 points) (1) In 2009, Changchun Institute of Applied Chemistry made a breakthrough in methanol fuel cell technology, the principle of which is shown in the figure below.

①Please, write down the electrode reaction formula that occurs when O2 is introduced from port C.

②Use graphite electrodes to electrolyze saturated brine. After the electrolysis starts, the surrounding area (fill in "cathode" or "anode") will first appear red. Assuming that the theoretical efficiency of the battery is 80% (the theoretical efficiency of the battery refers to the ratio of the maximum electric energy generated by the battery to the total energy released by the battery reaction), if 6.4g of methanol gas is consumed, the number of electrons passing through the external circuit is number).

(2) Industrial wastewater often contains heavy metal ions such as Cu2+, and direct discharge will cause pollution. At present, in the process of industrial wastewater treatment, according to the principle of precipitation transformation, insoluble substances such as FeS are commonly used as precipitants to remove these ions. It is known that Ksp(FeS)=6.3×10-?L-2 and Ksp(CuS)=1.3×10-?L-2 at room temperature.

Please use the ionic equation to explain the above impurity removal principle___.

(3) Industrially, to deal with the acidic industrial wastewater containing -, green alum (7H2O) is used to reduce the hexavalent chromium ions in the wastewater to trivalent chromium ions, and then add excess lime water to convert the chromium ions into trivalent chromium ions ion.  Cr(OH)3 precipitates.

① The ionic equation of the redox process is:

②At room temperature, the solubility product of Cr(OH)3 Ksp=1×10-32 mol4?L-4, the pH of the solution should be at least ____, in order to completely precipitate Cr3+.

③Using the above method to treat wastewater containing chromium (+6 valence) 78mg?L-1, the mass of green vitriol needs to be kg.  (keep the main calculation process)