[LucaZombini]

5g of a zinc/copper alloy are attacked with acid and 7dm³ of gas developed as a result. calculate the percentage of copper (standard conditions). I don't know the solutions, actually. It seems easy but i can barely start the problem.

n_tot = 7/22,7 = 0,308 mol.

i can't go further.

[AWK]

What acid? There are acids that react only with zinc, but there are also acids that react with zinc and copper.
In addition, 5 g of pure zinc will release with acid only 1.7 liters of gas.