Question:
How much KClO3 must be heated to obtain 2.50 gram of oxygen?
Answer: 
The equation of this reaction:
2KCIO3 → 2KCI + 3O2
For solving this question we can use proportion:
Molecular mass of KCIO3 = 122.5*2 = 245
Molecular mass of O2 = 32*3 = 96.
Suppose that x is mass of KCIO3. So, we get the proportion: x/245 = 2.50g/96 x = 6.38g of KCIO3


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