​What is mass of oxygen required to burn 2g of magnesium ribbon?how much oxide is formed?

Dear Student,

Mass of magnesium = 2 g
molar mass of Mg = 24 g/mol
Moles of Mg = mass/molar mass = 2/24 = 0.083 mol
2Mg + O₂ ---> 2MgO
2 moles of Mg react with 1 mole of O2. 
So, moles of O2 reacting = 0.0415 mol
Molar mass of O2 = 32 g/mol
mass of O2 reacting = number of moles*molar mass = 0.0415*32 = 1.33 g
2 moles of MgO are formed from 2 moles of Mg. So, same number of moles are being formed.
Moles of MgO = 0.083 mol
​Molar mass of MgO = 40 g/mo
Mass of MgO formed = ​number of moles*molar mass = 0.083*40 = 3.32 g

Regards