a) H2 + Cl2 = 2HCl
1mol H2.............2moli HCl
x=2mol................4moli HCl
b) N2 + 3H2 = 2NH3
3moli H2..........2moli NH3
x=.....................12 moli NH3
x=3*12/2
x=18 moli H2
c) C + 2H2 = CH4
2 moli H2...................16g CH4
x=...............................64g CH4
x=64*2/16
x=8 moli H2