2. Because the solubility of sulfide (except sodium, ammonia and potassium) in water is very small, and the solubility of arsenic sulfide is also very small, which is 18℃, 5.17×10 ∧-5g/100gH2O, so there are a lot of it. At this time, it is only the reaction of strong acid and weak acid with strong base salt (showing alkalinity). There are residual As3+, Na+ and SO42- in the water.
3. After adding hydrogen peroxide and CaO, Fe2+ is oxidized to Fe3+, and As3+ is oxidized to AS5+. The possible oxygen ions in water are Ca2+, Na+, H+ and As5+, and the possible anions are OH-, SO42-, AsO43- and trace S2-.
Na+ does not produce precipitation, so it is excluded, while the trace amount of S2- precipitation is very small, so it is not taken as the object of investigation. The aqueous solution is alkaline, and there will be little precipitation of H+, so it is not considered.
Therefore, the final precipitate to be considered is the combination of Ca2+ and Fe3+ with OH-, SO42- and AsO43-.
F3+ and SO42- do not generate precipitation, but the precipitation of Fe3+, OH- and AsO43- has been given in the problem, so the precipitation X in the problem will not be iron salt;
Ca2+ and OH- do not precipitate, and the precipitation with AsO43- has been given, so it is only possible for Ca2+ and SO42- to form a salt, namely CaSO4.