how many grams of kno3 should be used to prepare
2.50 l of a 0.500m solution
To start, I'm sure you meant M and not m. m stands for molality while M stands for molarity. That fact you want 2.50 L "of solution" tells me is must be M.
How many moles do you want? That's M x L = mols KNO3.
Then grams KNO3 = mols KNO3 x molar mass KNO3.
Post your work if you get stuck.