(1) According to the information in the title, Se reacts with concentrated HNO 3, Se is oxidized to +4 valence H 2 SeO 3, HNO 3 is reduced to NO and NO 2, and the ratio of the amount of substances producing NO and NO 2 is 1: 1, that is, the ratio of the two measurement coefficients is1:/kloc-. According to the conservation of electron transfer, the coefficient of se is1× 3+14 =1,so the reaction equation is: Se+2HNO 3 (concentrated) = H2SO4+NO+NO2 =, so the answer is: Se+2HNO 3 (.

(2) In the redox reaction, the oxidizability of oxidant is stronger than that of oxidation products, so according to the reaction equation, the order of the oxidizability of SeO 2, H 2 SO 4 (concentrated) and SO 2 from strong to weak is H 2 SO 4 (concentrated) > SEO 2 > SO 2, so the answer is: H 2 SO 4 (concentrated) > SEO 2 > SO 2;

(3) According to the reaction equation, the consumption of SEO 2 ~ 2I2 ~ 4Na2S2O3 is n(Na 2 S 2 O 3 )=0.2000? Mol/L×0.025L=0.005mol, and according to the relation, n (seo2) = 0.005mol×1.4 = 0.001.25mol in the sample. Therefore, the mass of SeO 2 is 0.00125mol×1g/mol = 0.13875g, so the mass fraction of SeO 2 in the sample is 0.13875g 0.2500g×/kl.