Question

How would you prepare 100 ml of 0.4 M MgSO4 from a stock solution of 2 M MgSO4?

Answer 1

OK, so to answer this question, you will simply use the molality equation which is as follows:
M1V1 = M2V2 
In the givens you have:
M1 = 2M
V1 is the unknown
M2 = 0.4M
V2 = 100 ml

plug in the givens in the above equation:
2 x V1 = 0.4 x 100 
therefore:
V1 = 20 ml

Based on this: you should take 20 ml of the 2 M solution and make volume exactly 100 ml in a volumetric flask by diluting in water.