(h) Two hydrogen atoms are preceded by the number V; Four carbon dioxide molecules are the number 4 before the chemical formula of carbon dioxide; Nine sodium ions are preceded by the number 9, and the valence of oxygen in water is -v. Write the chemical formula of water first, and then add-V to the symbol of oxygen; D particles that can best maintain the chemical properties of oxygen are oxygen molecules, so fill in: VH; 4COv9Na+； Hv？ vO； ov；

(V) ① As can be seen from the figure, at vi C, the solubility of substance A is v5g, and the mass ratio of solute to solvent in saturated solution is v5: HII = H: 4, so: v5, H: 4;

② At ②vi C, the solubility of substance 0 is v5g, and the mass fraction of solute in saturated solution is v5ghv5g×hii%=vi%, so fill in: VI%;

(9) Hydrometallurgy can be used, that is, iron can react with copper sulfate solution to generate ferrous sulfate and copper, so fill in: Fe+CuSO4 = Cu+FeSO4;

(4) Iron is easy to rust when it coexists with water and oxygen, so: water and oxygen.