Molarity is defined as the number of moles of solute in volume of 1 L solution. Molarity of NaCl is 4.2 M the number of NaCl moles present is 6.5 mol the volume of 4.2 moles is dissolved in 1 L solution therefore volume required for 6.5 mol is - 6.5 mol / 4.2 mol/L = 1.5 L therefore volume required is 1.5 L