In a certain mixture, one compound may be decomposed by heating, releasing 0.78 gram of oxygen. It is calculated that the oxygen makes up 21.5% of the compound. If the total mixture has a mass of 8 grams, what percentage of the mixture is the original compound that was decomposed?
The problem doesn't specify that the oxygen is completely discharged or not but I'm assuming it is. You may want to check with others to see how they interpret the question. The other point I'm assuming is that the other part of the mixture is inert.
.........A + MO ==> A + O2
.........x....y.....x...0.78
So MO must have 0.78g oxygen in it and if it is 21.5% of MO, then
0.215 = mass oxygen/mass sample
0.215 = 0.78/mass MO
mass MO = ?
8-mass MO = mass A
%MO = (mass MO/8)*100 = ?