Which is a weaker base, Cl or NO2

To find out which of the bases is weaker the only thing you need is to compare the atomic number of each element you got from the task. Then, the smaller number is obviously indicates the weaker base. So from the periodic table we know that atomic number of Cl is 17, and atomic number of No2 is should be about 35 which is enough to conclude that Cl has a weaker base.