in 8M of KNO3, there are x moles of KNO3 divided by (100/1000)L of water (because molarity is concentration, given by no. of moles divided by total volume in liters).
by changing the subject of formula, x = 0.1L × 8M = 0.8mol
given the no. of mol of KNO3, mass of KNO3 is given by no. of mol multipled by the relative molecular mass of KNO3.
mass = 0.8mol × 101.10g/mol (or whatever your periodic table says) = 80.88g