An element has two isotopes. one has a mass of 5.03 amu and an abundance of 25%, the other has a mass of 7.20 amu and an
abundance of 75%. what is the average atomic mass of the element?

Given the following at 25c calculate delta hf for hcn (g) at 25c. 2nh3 (g) +3o2 (g) + 2ch4 (g) > 2hcn (g) + 6h2o (g) delta h rxn= -870.8 kj. delta hf=-80.3 kj/mol for nh3 (g), -74.6 kj/mol for ch4, and -241.8 kj/mol for h2o (g)