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A crucible containing potassium chlorate has a mass is 24.426 g. The crucible is heated according to the lab procedure for this week's experiment and is found to weigh 23.202 g. Write an equation for the reaction and, assuming that all the chlorate decomposed, calculate the mass of the potassium chlorate originally present in the crucible?

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24.426g = mass xble + KClO3
23.202g = loss in mass upon heating
------------
1.224 g = mass O2 evolved.

2KClO3 ==> 2KCl + 3O2

mols O2 = grams/molar mass = 1.224/32 = about 0.038 (but you do it more accurately).
Convert to mols KClO3.
0.038 mol O2 x (2 mol KClO3/3 mol O2) = 0.038 x 2/3 = 0.026 mol KClO3.
g KClO3 = mols x molar mass = 0.026 x 122.6 = about 3.126 g = mass KClO3 originally present.

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