Problem 1

A solution was prepared from 0.5284 g of an alloy containing aluminium. The aluminium was precipitated as aluminium 8-hydroxyquinolate,

\ce{Al(C9H6NO)3}

. The precipitate was separated, dissolved in hydrochloric acid, and the liberated 8-hydroxyquinoline was titrated with a standard potassium bromate solution containing potassium bromide; 17.40 cm

^3

of the standard solution were required, and the resultant product is a dibromo derivative of 8-hydroxyquinoline. The relative atomic mass of aluminium is 26.98. 1.1 Write the equation for the reaction of

\ce{Al^{3+}}

with 8-hydroxyquinoline. 1.2 Give the name of the type of compound formed in the precipitation. 1.3 Write the equation in which bromine is produced from bromate and bromide. 1.4 Write the equation for the reaction of bromine with 8-hydroxyquinoline. 1.5 Calculate the molar ratio of aluminium ions to bromate ions. 1.6 Calculate the percentage by weight of aluminium in the alloy.

Step 2 of 4: Bromine generation and oxine bromination

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\ce{BrO3^- + 5 Br^- + 6 H^+ -> 3 Br2 + 3 H2O};\qquad \ce{C9H7NO + 2 Br2 -> C9H5Br2NO + 2 HBr}

Analysis

In acid the bromate oxidises bromide to $\ce{Br2}$ in situ ( $\ce{BrO3^- + 5Br^- + 6H^+ -> 3Br2 + 3H2O}$ , i.e. 1 mol $\ce{BrO3^-}$ furnishes 3 mol $\ce{Br2}$ ); the liberated $\ce{Br2}$ substitutes the two free ring positions of each oxine to give the dibromo derivative.