The recommended procedure for preparing a very dilute solution is not to weigh o

Question

The recommended procedure for preparing a very dilute solution is not to weigh out a very small mass or measure a very small volume of stock solution. Instead, it is done by a series of dilutions. A sample of 0.7751 g of KMnO4 was dissolved in water and made up to the volume in a 500.0-mL volumetric flask. A 2.000-mL sample of this solution was transferred to a 1000-mL volumetric flask and diluted to the mark with water. Next, 10.00 mL of the diluted solution was transferred to a 250.0-mL flask and diluted to the mark with water. (a) Calculate the concentration (in molarity) of the final solution. Enter your answer in scientific notation. -7 9.2 (b) Calculate the mass of KMnO4, needed to directly prepare the final solution. Enter your answer in scientific notation. -5 3.63

Explanation / Answer

Moles of KMnO4 in 500.0 mL flask = 0.7751 / 158.034

                                                      = 4.905 x 10^-3 mol

Moles of KMnO4 in 1000 mL flask = (2.00 / 500 ) x 4.905 x 10^-3

                                                      = 1.962 x 10^-5 mol

Moles of KMnO4 in 250.0 mL flask = (10.00/1000 ) x 1.962 x 10^-5

                                                       = 1.962 x 10^-7 mol

(a) Final concentration = moles/volume

                                     = 1.962 x 10^-7 / 0.250

Final concentration = 7.858 x 10^-7 M
                                         
(b) Mass of KMnO4 = Moles x molar mass of KMnO4

                                = 1.962 x 10^-7 x 158.034

                                = 3.101 x 10^-5 g

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