Which subatomic particles determine the mass of an atom?

Iknow the answer to 13 is b and 14 is d. i just need to know why the correct answers are correct

Nitrogen dioxide decomposes according to the reaction 2 no2(g) ⇌ 2 no(g) + o2(g) where kp = 4.48 × 10−13 at a certain temperature. if 0.70 atm of no2 is added to a container and allowed to come to equilibrium, what are the equilibrium partial pressures of no(g) and o2(g)

What is the chemical name of the compound ti2o3? use the list of polyatomic ions and the periodic table to you answer.