Question

What is the volume needed to make 4.2M solution using 6.5 mol NaCl?

Answer 1

Molarity is given as,

                             Molarity  =  Moles / Volume of Solution   ---- (1)

Given Data:
                   Molarity  =  4.2 mol.dm⁻³

                   Moles     =  6.5 mol

To Find:
                  Volume  =  ?

Solution:
              Solving Eq. 1 for Volume,

                              Volume  =  Moles ÷ Molarity
Putting Values,
                              Volume  =  6.5 mol ÷ 4.2 mol.dm⁻³ \

                              Volume  =  1.54 dm⁻³                    ∴ 1 dm³  =  1 L

Answer 2

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