To calculate the number of moles of NaOH needed to prepare 2 liters of a 2.0 M solution, we can use the formula:

Molarity (M) = Moles of solute (n) / Volume of solution (V)

Rearranging the formula to solve for moles of solute (n):

n = M x V

Substituting the given values into the formula:

n = 2.0 M x 2 L = 4 moles

Therefore, 4 moles of NaOH are needed to prepare 2 liters of a 2.0 M solution.