Question

when 3g of magnesium oxide is burnt in 2g Of oxygen ,5g of magnesium oxide is produced . what mass of magnesium oxide will be formed when 3g of magnesium is burnt in 5g of oxygen

Answer 1

5 g because magnesium reacts with oxygen with 3:2 ratio and extra oxygen or extra magnesium will remain unreacted.
In this case with 3 g of magnesium 2 g of oxygen react and other 3 g of oxygen remains un reacted

Answer 2

if 3g of Mg is burnt in 5g of O, then it will use up only 2g if O and give out 5g of MgO and 3g of O will be left out.
This answer is supported by the law of constant proportion