Question

You have an unknown dye solution whose concentration seems to lie between 0.0006M stock solution and a 0.0004 stock solution. Therefore, you had planned to dilute the 0.0006M stock to create your calibration standards. However, you see that there is no 0.0006M stock left but there is plenty of they 0.0004M stock

a. How could you raise the concentration of some of the 0.0004M stock to make 75mL of a 0.0006M solution without adding any more dye? Be sure to show calculations and indicate that volume of 0.0004M required.

b. Please draw a molecular view picture illustrating the moles of dye remaining constant during this process

Answer 1

Ans. Required solution = 75 mL of 0.0006 M solution

Number of moles of dye in 75 mL of 0.0006 M solution = Molarity x Volume in L

                                    = 0.0006 M x 0.075 L                  ; [1 L = 1000 mL]

                                    = 0.0006 moles L^-1 x 0.075 l        ; [1 M = 1 mol/ L]

                                    = 0.000045 moles

Thus, the required solution has 0.000045 moles of dye in 75 mL solution.

Preparation of Required solution: Since, the source solution (0.0004 M) available for preparing the required solution is diluted, it must be concentrated by boiling (or evaporation of the solvent).

Now,

Calculate the volume of 0.0004M solution that has 0.000045 moles of dye

Using, number of moles = Molarity x Volume in L

            Or, 0.000045 moles = 0.0004 M x V =

            Or, V = 0.1125 L = 112.5 mL

Hence, 112.5 mL of the solution has 0.000045 moles of dye.

Preparation: Carefully transfer exactly 112.5 mL of the 0.0004 M dye solution in a flat bottom flask (250 mL flask shall be sufficient).

            Gently heat the solution of get the volume reduced to around half of the initial volume (say, 60 ml).

            Allow the solution to cool to room temperature.

            Transfer it to a 75 mL graduated measuring cylinder. Make the volume upto 75 ml with distilled water. It is the required solution- 75 mL of 0.0006 M.

Note, it is best to make up the volume using standard volumetric flask. Since, 75 ml volumetric flask is difficult to find, do the calculation for 100 mL of 0.0006 M solution. Similarly, make 100 mL of 0.0006 M dye solution by boiling desired amount of 0.0004 M solution. Use 75 mL of 0.0006 M. It gives you much better accuracy.

Part B:

Number of moles of dye in 75 mL of 0.0006 M solution (Solution A) = Molarity x Volume in L

                                    = 0.0006 M x 0.075 L                 

                                    = 0.000045 moles

Number of moles of dye in 112.5 mL of 0.0004 M solution (solution B) =

                                    = 0.0004 M x 0.1125 L               

                                    = 0.000045 moles

Since, condensation of solution B by removing solvent does not causes loss of dye molecules, the condensed volume (75.0 mL) of the dye still has 0.000045 moles in it.

Both solution A and B have equal number of moles of the dye.