I’m assuming that this is based on percent mass.
Strontium Oxalate has a formula weight of 175.64 g/mol, and water has a f.w. of 18.02 g/mol. Because SrC2O4 makes up nearly 91% of the weight by mass, we can assume that the number of H2O molecules in this formula are very low.
You can test SrC2O4•H2O (f.w. 193.66) and the role that the strontium oxalate plays by mass by dividing the weight of the strontium oxalate by the weight of the strontium oxalate monohydrate. In this case:
175.64/193.66 = 0.9069, or 90.69% by weight.
Because the weight calculated here matches the one requested in the problem, SrC2O4•H2O, or strontium oxalate monohydrate, is the formula for the hydrate.
