(Original post by clownfish)
11) As there are two Br isotopes to consider and two Br in each molecule, you could have:
a) one with 79Br and 79Br
b) one with 79Br and 81Br
c) one with 81Br and 81Br
So you would have three peaks for the molecular ion.
There are often other peaks present around the molecular ion for an organic molecule (eg for when some carbon 13 is present), but the abundance of other isotopes is so small that these peaks are only tiny. For bromine the two isotopes have a bigger abundance relative to each other, hence you notice the peaks due to differing isotopes.
12) Remember it's (sum of Mr of useful products/sum of Mr of all products)x100
and make sure you've got the correct product as your useful product (a very easy mistake to make on these). It does work.