Question

To prepare a 0.400 M NaCl solution, add _______ of NaCl and enough water to form a solution with a volume of _______ in a volumetric flask.

answer is 5.84 g, 250 mL

Answer 1

We know that MOlarity , M = number of moles(n) / Volume of solution in L

Given Molarity of NaCl is , M = 0.400 M

                                          = 0.400 mol / L

So the number of moles of NaCl in 1L = 0.400 mol

The number of moles of NaCl in 250 mL of the solution = ?

    1 L(=1000mL) ----> 0.400 mol

               250 mL -----> M(=?)

M = ( 250x0.400) / 1000

      = 0.1 mol

We know that molar mass of NaCl = At.mass of Na + At.Mass of Cl

                                                   = 23+35.4

                                                  = 58.4 g/mol

Also number of moles , n = mass/molar mass

So mass of NaCl , m = number of moles x molar mass

                                = 0.1 mol x 58.4 (g/mol)

                                = 5.84 g

Therefore exactly weigh & transfer 5.84 g of NaCl into a 250 mL of the flask & dissolve it by adding water we get 0.400 M NaCl solution.