Answer:
A pure substance is a form of matter that has a constant composition and properties that are constant throughout the sample. Mixtures are physical combinations of two or more elements and/or compounds.
C3H8 -----> 3 CO2 (from equation)
1mol C3H8 ------ 3 mol CO2
2.0 mol C3H8 ---- x mol CO2
x=(2.0 mol C3H8* 3 mol CO2)/1mol C3H8 =6 mol CO2
B:Molecules must collide.
Why: They have to collide so the chemical bonds can break.