How many grams of potassium dichromate should be added to a 50 mL flask to prepare a 0.02505 M solution when the flask is diluted to the water with water?

You want a 0.02505 molar (moles/liter) solution. You will have 50 mL, so how many total moles of potassium dichromate is that?

0.050 L * 0.02505 moles/L = 0.0012525 moles

Now, how much does a mole weigh? Well, what is the formula for potassium dichromate (look it up and calculate the molecular weight). Then, multiply the molecular weight (in grams/mole) by the number of moles (g/mole * mole = g)