In order to prepare 1.00 l 1.00 mol / L NaCl solution, 58.5 g NaCl can be dissolved in 1.00 L water

In order to prepare 1.00 l 1.00 mol / L NaCl solution, 58.5 g NaCl can be dissolved in 1.00 L water

The error lies in "dissolving in 1 L water". The volume of 58.5 g NaCl dissolved in 1 L water is no longer 1 L, and the formula of mass concentration of substance must be divided by the volume of solution, not the volume of solvent

To prepare 1.00l, 1.00mol × L ^ - 1 NaCl solution, 58.5g NaCl can be dissolved in 1.00l water, right

Wrong, the volume of solution will change after dissolution, the volume will not be 1 L, so the concentration is not 1 mol / L
It is correct that 58.5g NaCl should be prepared into 1L solution
n=m/M=58.5/58.5=1mol
c=n/V=1/1=1mol/L

What is the experimental plan for preparing 100 ml 0.5 mol / L sodium chloride solution with 2.1 mol / L sodium chloride solution

100 ml (0.100 L) 0.5 mol / L sodium chloride solution containing sodium chloride: 0.100 * 0.5 = 0.050 mol / L
2.1 mol / L sodium chloride solution required: 0.050 / 2.1 = 0.024 L = 24 ml
Take 24 ml 2.1 mol / L sodium chloride solution and add water to 100 ml