[sarahjane]

how do you calculate the amount of potassium dichromate required to make 250mLs of a 0.15M solution

[gregpawin]

Simply enough, molarity is just the measure of the number of moles of a solute in liters of a solvent.  So molarity (M) is equal to the number of moles of solute divided by the liters of solvent.  M=n/V where n are moles of solute and V is the number of liters of solvent.  There are three variables here.  The problem has given you two of the variables, so you can solve it.  0.15M=n/0.250L so n = 0.0375moles of K₂Cr₂O₇.

I'm probably sure they'll want you to convert it into grams so just multiply it by the molar mass.