Calculate the mass of Na2S2O3•5H2O needed to prepare 250.0 ml of 0.2000 M solution. Note, include the 5 waters in your calculation of the formula mass.

Answer 1

How many moles do you need? That's moles = M x L = 0.2000 x 0.2500 = ?
Then grams Na2S2O3.5H2O = moles x formula mass