Since we have 2 reactants, the question already states that water is the excess reagent. Therefore, the amount of H2 gas formed is dependent on the mass of sodium (Na; 8.0 g) that you start with.
Since the reaction is already balanced, convert mass of Na to moles of Na using its molar mass (23 g/mol).
8.0 g x (1 mol Na / 23 g Na) = 0.35 mol Na (rounded to 2 significant figures)
Next, convert mol Na to mol H2 using the mole ratio from the balanced reaction. From the reaction, you can see that it takes 2 mol Na to form 1 mol H2.
0.35 mol Na x 1 mol H2 / 2 mol Na = 0.18 mol H2
Finally, convert mol of H2 to g of H2 using its molar mass (2 g/mol)
0.18 mol H2 x 2 g H2 / 1 mol H2 = 0.36 g H2