Practice Problem 1
Determine the oxidation number of each element in the following compounds:
(a) BaO2 (b) (NH4)2MoO4 (c)Na3Co(NO2)6 (d) CS2
(a) If the oxidation number of the oxygen in BaO2 were -2, the oxidation number of the barium would have to be +4. But elements in Group IIA can't form +4 ions. This compound must be barium peroxide, [Ba2+][O22-]. Barium therefore is +2 and oxygen is -1.
(b) (NH4)2MoO4 contains the NH4+ ion, in which hydrogen is +1 and nitrogen is -3. Because there are two NH4+ ions, the other half of the compound must be an MoO42- ion, in which molybdenum is -6 and oxygen is -2.
(c) Sodium is in the +1 oxidation state in all of its compounds. This compound therefore contains the Co(NO2)63- complex ion. This complex ion contains six NO2- ions in which the oxidation number of nitrogen is +3 and oxygen is -2. The oxidation state of the cobalt atom is therefore +3.
(d) The most elecronegative element in a compound always has a negative oxidation number. Since sulfur tends to form -2 ions, the oxidation number of the sulfur in CS2, is -2 and the carbon is +4.