A granulated sample of aircraft alloy (Al, Mg, Cu) weighing 8.72g was first reacted with alkali and then with very dilute HCl, which left behind a residue. The residue after boiling with alkali weighed 2.10 g and the acid-insoluble residue weighed 0.69g. what is the composition of the alloy?

Explanation

Let us suppose, the amount of Al, Mg and Cu in the sample be x, y and z g respectively.
Reaction: `2Al+2NaOH overset(2H_(2)O) to 2NaAlO_(2)+3H_(2)`
`Mg+2HCl to MgCl_(2) +H_(2), Cu+HCl to `No reaction
i.e., only Al reacts with NaOHH and only Mg reacts with HCl.
`therefore x+y+z=8.72 and y+z=2.10`
(Residue left after alkali treatment)
z=0.69 (Residue left after acid treatment)
`x=6.62g and y=2.10-0.69=1.41g`
`therefore%` of `Al=(6.62)/(8.72)xx100=75.9`
% of `Mg=(1.41)/(8.72)xx100=16.2`
% of `Cu=(0.69)/(8.72)xx100=7.9`