Answer: 448 mL of hydrogen gas.
Solution:

Now,according to reaction
1 mole of magnesium produces one mole of hydrogen gas.
Then 0.020 moles of magnesium will produce 0.020 moles of hydrogen gas.
Moles of
gas : 0.020 mol
At STP, 1 mol of gas occupies 22.4 L
So volume of 0.020 mol of
produced at STP :
= 22.4/times 0.020 = 0.448 L = 448 mL (1L=1000mL)