\(a) Mg + H_2SO_4 \to MgSO_4 + H_2\\ n_{H_2} = n_{H_2SO_4} = \dfrac{4,9}{98} =0,05(mol)\\ V_{H_2} = 0,05.22,4 = 1,12(lít)\\ b) Mg + 2HCl \to MgCl_2 + H_2\\ n_{H_2} = \dfrac{1}{2} n_{HCl} = \dfrac{1}{2} . \dfrac{4,9}{36,5} = \dfrac{49}{730} (mol)\\ V_{H_2} = \dfrac{49}{730} .22,4 = 1,504(lít) > 1,12\\ \text{Suy ra, thể tích Hidro thay đổi.}\)