Posted by Ana on Tuesday, March 2, 2010 at 10:23pm.
2KClO3 > 2KCl + 3O2
What mass of O2 can be made from heating 125 g of KClO3?
I know that the answer is 49 g O2 but I keep getting a completely different answer, can anyone show me what I'm doing wrong?
125g KClO3 x (2 mol KClO3/ 122.49g KClO3) = 2.04 mol KClO3
2.04 mol KClO3 x (3 mol O2/1 mol KClO3) =6.12 mol O2
6.12 mol O2 x(31.98g O2/1 mol O2) = 195.71 g O2

Chemistry  DrBob222, Tuesday, March 2, 2010 at 10:45pm
125g KClO3 x (2 mol KClO3/ 122.49g KClO3) = 2.04 mol KClO3
I think the problem is the method. moles KClO3 = grams/molar mass = 125/122.5 = 1.02 moles.
2.04 mol KClO3 x (3 mol O2/1 mol KClO3) =6.12 mol O2 Note here that the equation tells you 3 mol O2 = 2 mol KClO3 AND you have the extra 2 thrown in for KClO3 also.
Now convert mols KClO3 to moles O2.
moles O2 = moles KClO3 x (3 moles O2/2 moles KClO3) = 1.02 x 3/2 = 1.5306
6.12 mol O2 x(31.98g O2/1 mol O2) = 195.71 g O2
Then 1.5306 moles x (32 g/mol) = 48.98 g which rounds to 49.0 to three s.f.
I think you multiplied by the 2 for 2KClO3 when in reality further down the line I divided by 2. So your answer is high by 2 x 2 = 4. Note 196/4 = 49.00. There is a way to do it with the 2KClO3 if you want to see but the method I showed you is the more standard one I see in all of the texts.