Exam Style Question - 1

Question: Deduce the oxidation state of chromium in Cr₂O₇^2- and of sulfur in SO₄^2-.Rules: oxygen is -2 in most compounds, and the sum of the oxidation states in an ion equals the charge on the ion.Cr₂O₇^2-: seven oxygens give 7 × (-2) = -14. The two chromiums must total -2 - (-14) = +12, so each chromium is +6.SO₄^2-: four oxygens give -8. Sulfur must be -2 - (-8) = +6.When chromium changes from +6 in Cr₂O₇^2- to +3 in Cr³⁺, its oxidation state decreases, so chromium is reduced.