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Question Q161) 1.0 g of magnesium is burnt with 0.56 g O2 in a closed vessel. Which reactant is left in excess and how much? (At. wt. Mg = 24; O = 16) [AIPMT-2014]


Reaction is
2Mg + O2 → 2MgO
Two moles of Mg reacts with one mole of oxygen to produce 2moles of MgO
1.0g of Mg = 1/24 moles = 4/96
0.56 g of O2 = 0.56/32 moles = 1.68/96
For 1.68/96 moles of oxygen Mg requires = 2(1.68/96)
Mg moles taken is 4/96 which is more than complete reaction of O2
Excess moles Mg=4/96 -(2×1.68)/96=0.64/96
Excess weight of Mg= Excess mole × molecular wt
Excess weight =0.64/96×24=0.16g
Answer : (a)