If matter is neither created nor destroyed, why can’t we just go directly from grams of reactant to grams of product?

Which substance can be broken down by chemical means

Before it was launched, a helium-filled balloon had a pressure of 201 kpa at a temperature of 27°c. at an altitude of 15,000 m, the pressure had decreased to 2.5 kpa and the temperature had dropped to -14 °c. the volume of the balloon increased to 59.3 m3. what is the original volume of the balloon? 13 m3 0.85 m3 0.077 m3 1.17 m3

According to thomson, the atom is a positively charged cloud with electrons scattered throughout. what would the alpha particles do when they hit the foil if thomson were correct