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KClO3(s) produces KCl and O2 when heated. 2.34L of O2 was collected over water at 9878mmHg at 65°C. What mass of KClO3 reacted?

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J.R. S. | Ph.D. in Biochemistry--University Professor--Chemistry TutorPh.D. in Biochemistry--University Profes...
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2KClO3 ==∆==> 2KCl + 3O2   ...   balanced equation
Assuming the pressure is 987.8 mm Hg (and not 9878 mm), then..
Find moles of O2 produced. 
PV = nRT
P = 987.8 mm Hg - 187.5 mm Hg = 800.3 mm Hg (the 187.5 mm Hg that was subtracted is the vapor pressure of water at 65ºC)
V = 2.34 L
R = gas constant = 62.36 Latm/Kmole
T = 65 + 273 = 338K
n = PV/RT = (800.3 mm)(2.34 L)/(62.36 Lmm/Kmol)(338K)
n = 0.0889 moles O2
Moles KClO3 that reacted = 0.0889 moles O2 x 2 moles KClO3/3 moles O2 = 0.0593 moles KClO3
Mass KClO3 that reacted = 0.0593 moles x 122.6 g/mole = 7.27 g (3 significant figures)