Please help my teacher didn’t teach us this and I can’t find the answers!! (Physical Science)

Calculate the number of molecules present in 0.750 mol of mgo.

Which statement is usually true about the relationship between activation energy and reaction rates? low activation energy barriers result in low rates. high activation energy barriers result in low rates. low activation energy barriers result in no reaction. high activation energy barriers result in no reaction.

When a beta particle is emitted, the mass number of the nucleus a. decreases by one b. increases by one c. remains the same d. decreases by two