14,5% of Zn
From the gasses ideal Law and by consedering that 760 mmHg = 1 atm we have
These mol of
from which you see that need 1 mol of Zn to form 1 mol of
If 1 mol of atoms of Zn weigh 65,4 g, 0,0133 mol of Zn weigh 0,872 g that in 6 g of alloy are 14,5% of the brass.
That is a theoretical reaction because if Zn react quite quantitatively with sulfuric acid, Cu could reacts partially especially if sulfuric acid is warm and concentrated.