The minimum combined kinetic energy reactant particles must possess in order for their collision to result in a reaction is called the

Activation energy

Explanation:

In a chemical reaction, the reactants contains particles which must collide in order for a reaction to occur. The rate of reaction depends on the frequency of effective collision between the reacting particles. Effective collision are those that result in reactions, which when they occur the colliding particles become activated with increased kinetic energy.

This energy must exceed a particular energy barrier for a particular reaction if the reaction must take place. This energy barrier that must be overcome before a reaction takes place is known as the ACTIVATION ENERGY.

To explain further, when two particles or molecules A and B come in contact with each other, for a reaction to take place, they must collide with a sufficient force to break the bond that exists between them. The minimum combined kinetic energy these reactant particles must possess in order for their collision to result in a reaction is called the activation energy.

the formula mass of a molecule or a compound is the sum of the atomic weights of the atoms in its empirical formula

(that's what i know about formula mass)

you need to have a picture of the atom that way we can answer it