There are 1000 mililiters in a liter, so 1000 ml for every liter, you have 5 liters, so:
5L*1000 = 5000 mL
1 Hydrogen 1s1
2 Helium 1s2
3 Lithium 2s1
17) 8.4 / 20 x 100
18) 20 . 0.5150
19) 6,50% because (as you said) the law of definite proportions states that regardless of the amount, a compound is always composed of the same elements in the same proportion by mass
