Why are some isotopes stable and others not stable

As a part of an experiment a student burns propane to produce carbon dioxide and water she remembers that she must follow the law conservation of matter when writing a balanced chemical equation which of these equation adheres to the law of conservation of matter

Write the empirical chemical formula of calcium with a mass percent of 38.8, phosphorus with a mass percent of 20.0, and oxygen with a mass percent of 41.3.

Which of the following units is not an official si unit? mole liter kilogram ampere