Hi, @sophiag12! So first you need to compute how many moles of you will need in a 500ml solution. So if the target concentration is 0.155 M just multiply that with volume (500ml or 0.5 L) you will get 0.0575 mol. Now using the molar mass (312.7421 g/mol) you can compute the amount in grams. Just multiply the moles with MM to get the mass which 17.9827g. I hope this helps!