a 2.4375 g sample containing a mixture of KClO3 + NaCl is heated and undergoes a mass loss of 0.6191g. What percent of KClO3 is in the mixture?

2KClO3>>2KCl + 3O2

ok, nothing happened to the NaCl. So the change was entirely to the KClO3

moles O2: .6191g/32= you do it.
moles KCLO3: same as above, times 2/3

mass of KClO3: Moles potassium chlorate, times mole mass.

percent KClO3: massKClO3/2.4375