NBS is used in place of Br2 during bromination because the use of Br2 tends to result in the formation of a large concentration of the addition product. This is because, the Br2 interacts with the double bond to yield only the dibromide (addition product).
The usefulness of NBS lies in the fact that it provides a lower concentration of Br2, and such that the rate addition is slower than the chain propagation steps. Only the monobromination product is observed
The low concentration of Br2 allows the free-radical reaction to out-compete the alkene addition reaction.
The molar mass of CH4 is about 16 grams per mol. So you get the answer by mutlitplying 0.5 by 16, (cancel out moles by placing 1 on the bottom of the fraction).
