How will you prepare 250mL of a 0.100M potassium permanganate solution?

From a practical point of view…Get a clean dry 250 mL volumetric flask. As far as the potassium permanganate goes, you will need 0.025 mole. This is because if you dissolve 0.025 mole in 250 mL, you will get a 0.1 M solution. The molecular weight of potassium permanganate is 158.034 g/mole. 0.025 mole, then, is 3.95 g (= 0.025 * 158.034) of potassium permanganate.

OK, so , now dump the 3.95 g of potassium permanganate into the 250 mL volumetric flask. Fill it water to about 3/4 full and shake until the permanganate dissolves. Then, fill to the mark with water. Invert the flask a few times and you are done.

EDIT: As pointed out by Jaleel Ali, the volumetric flask does not have to be dry. As long as it is clean and any liquid in it is water, it will not affect the concentration. However, if you are in the habit of rinsing glassware, after it is washed, with ethanol or acetone, residual solvent could be a problem. If you did not clean the glassware and see liquid in it, either dry it or rinse the volumetric with water a few times.

A concentration of 0.05M means:

1000ml contains 0.05 mols of KMnO4

Therefore 250ml should contain 0.05/1000 x 250 = 0.0125mol

You will take this amount (in mols) of KMnO4 from the original stock which has a concentration of 2M, meaning:

2mol KMnO4 is found in 1000ml

Therefore, your required amount, which is 0.0125mol is found in :

1000 /2 x 0.0125 = 6.25ml

• You measure 6.25ml of the stock solution, (preferably with a graduated pipette for more precision) and transfer it to a volumetric flask of 250ml.
• Then you make up to the mark with distilled water (which means you would add 243.75 ml of water- but you don’t need to measure that as making up to the mark automatically allows you to obtain exactly 250ml of solution).