If the decomposition of potassium chlorate is incomplete assuming all measurements are accurate, what would be the effect on the calculated value for the molar volume of oxygen at STP?

Answer 1

2KClO3>>2KCL + 3O2
you wont get as much gas, and if you assumed all the mass of KCLO3 had been consumed, and you measured the mass of KCL (you thought), you would actully have less KCl and some KCLO3 left, which means your calculated values of the mass of O2 (ie, massKClO3 minus massKCL) is too high: Example
initial mass KClO3=10g
in reaction that went to completion, Mass KCl=8 grams
O2 mass: 2 grams
but in your case
MassKCLO3=10g
say only 8 grams went in the reaction, so mass left over 2 grams, plus
KCl residue (4/5*8=6.4 grams) or total measure of 8.4 grams residue, but how much O2 was actually produced: (2/10*.8)=1.6 grams
So you in this incomplete reaction, measured too much residue, and you thought you had consumed 10 grams (but only 8.4), so your calculated O2 would be much too high.