Aflask contains 0.100 mol of liquid bromine, br2. determine the number of bromine molecules present in the flask.

Avagadros number is where 1 mol of any substance is made of 6.022 x 10²³ units. These units could be atoms making up an element of molecules making up a compound.
1 mol of Br₂ is made of 6.022 x 10²³ molecules of Br₂
the flask contains 0.100 mol
Therefore if 1 mol has 6.022 x 10²³ molecules of Br₂
then 0.1 mol has - 6.022 x 10²³ molecules/mol x 0.1 mol 
therefore number of molecules - 6.022 x 10²² Br₂ molecules ₂