How many atoms are present in 34.69 moles of mg

Avogadro's number. 

1 mole of any substance, molecule or element is equal to a certain amount of atom. 

6.022 x 10^23 is the Avogradro's constant. 

Magnesium Oxide is a compound. therefore if you have 30.3 g of it (1 mol), it will have the same number of atoms. 

34.69 moles of MgO has 208.9 x 10^23 number of atoms. 

2.089 x 10^25 is also a correct answer.