How many milliliters of a 4.0 m nacl solution will be needed to make 500 ml of a 2.0 m nacl solution?

The number of moles = volume * molarity.
  Volume = 500/1000 of Nacl solution.
 Volume = 0.5 of Nacl
  Hence number of moles = 0.5 * 2 = 1.
 To calculate how many milliliters of a 4.0 m nacl solution.
 We have number of moles = volume * molarity.
 Hence volume = number of moles /molarity
  Volume = 1 / 4 = 0.25L = 2500mL

Anions are negative charged ions