Idon't know which the correct answer it is

Find the mass, in grams, of 5.00*10^23 molecules of f2

4.25g sample of solid ammonium nitrate dissolves in 60.0g of water in a coffee-cup calorimeter, the temperature drops from 22.0 c to 16.9 c. assume that the specific heat of the solution is the same as that of pure water. calculate delta(h) (in kj/mol nh4no3) for the solution proces.

How are isotopes of the same element alike?