A granulated sample of aircraft alloy `(Al, Mg, Cu)` weighing `8.72 g` was first treated with alkali and then with very dilute `HCl`, leaving a residue. The residue after alkali boiling weighed `2.10 g` and the acid insoluble residue weighed `0.69 g`. What is the composition of the alloy?

Answer

`Al=75.9%, Mg=16.2%, Cu=7.9%` ;