First find the mass of solute:
Molar mass KNO₃ = 101.1032 g/mol
mass = Molarity * molar mass * volume
mass = 0.800 * 101.1032 * 2.5
mass = 202.2064 g of KNO₃
To prepare 2.5 L (0800 M) of KNO3 solution, must weigh 202.2064 g of salt, dissolve in a Beker, transfer with the help of a funnel of transfer to a volumetric flask, complete with water up to the mark, capping the balloon and finally shake the solution to mix.
hope this helps!