how would you prepare a 500 mL of 0.020M KMnO4 solution?

Answer 1

molarity = # moles/liter of solution.
So how many moles do we want for 500 mL of 0.02 M KMnO4? We want
M x L = moles = 0.02 x 0.5 L = 0.01 moles.
moles = grams/molar mass.
You know moles and molar mass, calculate grams.

Answer 2

1.5803