1. Convert mL to L:

* 250 mL * (1 L / 1000 mL) = 0.250 L

2. Calculate moles of NaOH:

* Moles = Molarity * Volume (in L)

* Moles = 5.00 M * 0.250 L = 1.25 moles NaOH

3. Determine moles of hydroxide ions (OH⁻):

* NaOH dissociates completely in solution, producing one mole of OH⁻ for every mole of NaOH.

* Therefore, there are 1.25 moles of hydroxide ions (OH⁻) in the solution.