I don't know what experiment you are doing with the KMnO4 but in general solutions of unknown concentration must be standardized prior to use to determine their exact concentration so that the proper amount can be used in the experiment. Some chemical reactions are very sensitive to the exact amount of reagents used otherwise the reaction will not work properly. When you have a solution of unknown concentration and add 5 mL to the reaction you have no idea how many grams or moles of chemical you added (concentration is measured in mass per unit volume). Fortunately the concentration of a solution of many reagents can be found using a process called titration. When titrating a chemical you can determine its concentration by adding it to a known amount of another chemical until a color change happens. Then you can do the math and calculate the exact concentration of a KMnO4 solution. Once the exact concentration has been determined, the solution is said to be standardized and you can then add exact amounts of the KMnO4 to a chemical reaction.