Regents Chemistry Exam August 2022
When a neutron is absorbed by a uranium-235 nucleus, the nucleus can split. One possible nuclear reaction is represented by the balanced equation below.
In this reaction, the products have a mass that is 0.180 u less than the mass of the reactants.
84 Compare the energy released per gram of reactant during this reaction to the energy released per gram of reactant in a chemical reaction. 
Nuclear reactions fission and fusion release a lot of energy. There are even nuclear plants created to use the energy from the nuclear reactions to generate electricity.
Answer: The nuclear reaction releases more energy per gram of reactant than a chemical reaction.
85 Determine the time required for an 8.00-mg sample of Sr-90 to decay until only 2.00 mg of the sample remains unchanged. 
Let's take a look at Table N on the Reference table which gives half lives for selected radioisotopes.
Sr-90 has a half life of 29.1 years. That means that every 29.1 years its amount halves.
If we start with 8mg, in 29.1 years we would have half of that which is 4mg, and in another 29.1 years we would have 2mg.
Answer: 58.2 years.