Answer is: 3,94 mol of hydrogen gas.
Chemical reaction: 2K + 2HBr → 2KBr + H₂.
n(K) = 9,87 mol.
from reaction: n(K) : n(H₂) = 2 : 1.
9,87 mol : n(H₂) = 2 : 1.
n(H₂) = 4,935 mol at 100% of reaction.
n(H₂) = 4,395 mol · 0,798 = 3,938 mol.
n - amount of substance.