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?

Options

A. Weight % of `AI = 75.9%`
B. Weigth % of `Cu = 7.9%`
C. Weight % of `Mg = 16.2%`
D. All the above

Answer

D

Explanation

Let `Al,Mg` and Cu be a,b and c g respectively.
`2AI + 2NaOH rarr 2NaAIO_(2) +3H_(2)`
`Mg +2HCI rarr MgCI_(2) +H_(2)`
`Cu +HCI rarr` No reaction
i.e., only AI reacts with `NaOH` and then only Mg reacts with `HCI :. a + b +c = 8.72`
`b +c = 2.10` (Residue left afte alkali treatment)
`c = 0.69` (Residue left after acid treatment)
`:. b = 2.1 - 0.69 = 1.41 g`
`a = 8.72 - 2.1 = 6.62g`
`:. %` of `AI = (6.62)/(8.72) xx 100 = 75.9`
`%` of `Mg = (1.41)/(8.72) xx 100 = 16.2`
`%` of `Cu = (0.69)/(8.72) xx 100 = 7.9`