To calculate the number of grams of KNO3 required to prepare 500 mL of 1.20 M KNO3 solution, we need to use the formula:

Molarity (M) = moles of solute / volume of solution in liters

We can rearrange this formula to find the number of moles of solute needed:

moles of solute = Molarity (M) x volume of solution in liters

We know the volume of solution (500 mL = 0.5 L) and the desired molarity (1.20 M), so we can plug these values into the formula:

moles of KNO3 = 1.20 M x 0.5 L

moles of KNO3 = 0.60 moles

Now that we know the number of moles of KNO3 needed, we can use its molar mass to convert moles to grams:

mass of KNO3 = moles of KNO3 x molar mass of KNO3

The molar mass of KNO3 is 101.1 g/mol. Therefore:

mass of KNO3 = 0.60 moles x 101.1 g/mol

mass of KNO3 = 60.66 g

So, 60.66 grams of KNO3 will be required to prepare 500 mL of 1.20 M KNO3 solution.