Question

1 gm of magnesium is burnt with 0.56gm of O2 in a closed vessel.Which reactant is left in excess and how much

Answer 1

(At. wt. Mg = 24 ; O = 16)
The equation is as follows:

2Mg + O2 ------> 2MgO
(s) (g) (s)

Therefore 2 moles of Mg require 1 mole of O2 to produce 2 moles of MgO.

Calculate actual moles.
moles= mass/ molar mass

M g= 1/24 = 0.041667
O2 = 0.56/32 = 0.0175

Mole ratio of Mg: O2 = 2:1

find mass of Mg that reacts:

that means 2 mole of Mg reacts with 1 moles of O2

If 1 = 2
Then 0.0175 = 0.0175 x 2
= 0.035 moles of magnesium

mass of magnesium that reacts:

mass = moles x molar mass
= 24 x 0.035
= 0.84 g
That means that of the 1 gram of magnesium that reacts only 0.84 grams is used

Therefore magnesium is in excess and by 0.16