Question

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

Answer 1

125g KClO3 x (2 mol KClO3/ 122.49g KClO3) = 2.04 mol KClO3

<b>I think the problem is the method. moles KClO3 = grams/molar mass = 125/122.5 = 1.02 moles.</b>

2.04 mol KClO3 x (3 mol O2/1 mol KClO3) =6.12 mol O2 <i>Note here that the equation tells you 3 mol O2 = 2 mol KClO3 AND you have the extra 2 thrown in for KClO3 also.</i>

<b>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</b>

6.12 mol O2 x(31.98g O2/1 mol O2) = 195.71 g O2

<b>Then 1.5306 moles x (32 g/mol) = 48.98 g which rounds to 49.0 to three s.f.</b>
<i>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.</i>