The number of moles of HNO3 needed to prepare 5.0 liters of a 2.0 M solution can be calculated using the formula:

Molarity (M) = Moles of Solute (mol) / Volume of Solution (L)

Rearranging the formula to solve for moles of solute:

Moles of Solute (mol) = Molarity (M) * Volume of Solution (L)

Substituting the given values:

Moles of HNO3 = 2.0 M * 5.0 L = 10 moles

Therefore, 10 moles of HNO3 are needed to prepare 5.0 liters of a 2.0 M solution of HNO3.