Question

1g of mg burnt in a clossed vessel containing 0.5 grams of oxygen.find the reactant left in excess and what is weight of the reactant left?

Answer 1

Mg +1/2 O2 --->Mg0
moles of Mg = 1/24 = 0.04
moles of 1/2O2 = 0.5/16 = 0.03
Therefore Mg is left in excess as 0.01 moles

Answer 2

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
Mg=    1/24  = 0.041667O2=   0.56/32 = 0.0175MgO=
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 gThat 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