Consider the thermal decomposition of potassium chlorate.

Question

2 KClO3 (s)-----> 2 KCl (s) + 3 O2 (g)
A mixture containing KCl and KClO3 weighing 1.80 g was heated producing
1.40 x 102 mL of O2 gas at STP. What percent of the original mixture was KClO3?
Is this how I should approach this problem? By using the number of moles?
nmix=nkclo3+nkcl

Answer 1

That might work but it's the long way around AND you don't know mols KCl in the mixture, although you can calculate mols KCl but only after you've worked the problem.
I would use the equation you have to convert mL O2 gas to mols O2 and convert that to mols KClO3. Convert that to grams KClO3. Then
%KClO3 = (g KClO3/mass sample) = ?

Answer 2

Got it. Thank you so much