Potassium bromate, KBrO3, is a primary standard for the generation of Br2 in acidic solution:
BrO-3 + 5Br- + 6H+ ⇌ 3Br2 (aq) + 3H2O
The Br2 can be used to analyze many unsaturated organic compounds. Al3+ was analyzed as follows: An unknown was treated with 8-hydroxyquinoline (oxine) at pH 5 to precipitate aluminum oxinate, Al(C9H6ON)3. The precipitate was washed, dissolved in warm HCl containing excess KBr, and treated with 25.00 mL of 0.020 00 M KBrO3.




The excess Br2 was reduced with KI, which was converted into I-3. The I-3 required 8.83 mL of 0.051 13 M Na2S2O3 to reach a starch end point. How many milligrams of Al were in the unknown?