The required is the amount of Sr(OH)2·8H2O needed to prepare a solution having a concentration of 0.100 M OH-. We start with the concentration of the hydroxide ion to determine the number of moles hydroxide ion for a given volume of 250.0 mL solution.
0.100 mol/L (0.250 L) = 0.4 mol hydroxide ion needed.
We know that, for every mole of Sr(OH)2·8H2O in solution, two hydroxide ions is formed. Thus,
0.4 mol OH- ( 1 mol Sr(OH)2·8H2O / 2 mol OH-) ( 265.76 g / mol) = 53.152 g
Thus, 53.152 grams of Sr(OH)2·8H2O is needed to make a solution with 0.100 M OH-.