Approx. 16%16% with respect to the starting mass of potassium nitrate.
Explanation:
We assume that (under fierce heating), "potassium nitrate"potassium nitrate decomposes to give stoichiometric "potassium nitrite"potassium nitrite and "dioxygen gas"dioxygen gas:
KNO_3(s) + Delta rarr KNO_2(s) + 1/2O_2(g)uarr
And thus 1mol of "potassium nitrate" loses 16*g of oxygen gas upon decomposition. With respect to KNO_3, this represents a mass loss of: