Verified answer

2 KClO3 → 2 KCl + 3 O2

n = PV / RT = (725 torr) x (0.290 L) / ((62.36367 L torr/K mol) x (128 + 273) K) = 0.0084074 mol O2

(0.0084074 mol O2) x (2 mol KClO3 / 3 mol O2) x (122.5495 g KClO3/mol) = 0.687 g KClO3