Think about what you have here - you know that the original volume of the solution was
To get the number of moles of calcium sulfate, use its molar mass. A compound's molar mass tells you what the mass of one mole of that compound is.
In this case, calcium sulfate has a molar mass of
This means that the
#0.23color(red)(cancel(color(black)("g"))) * "1 mole CaSO"_4/(136.14color(red)(cancel(color(black)("g")))) = "0.001690 moles CaSO"_4#
You now know the number of moles of solute and the volume of the solution, which means that its molarity will be - do not forget to convert the volume to liters!
#c = n/V#
#c = "0.001690 moles"/(100 * 10^(-3)"L") = color(green)("0.017 M")#
I left the answer rounded to two sig figs.
SIDE NOTE At room temperature, calcium sulfate has a solubility of about 0.21 g per 100 mL of water, so your solution is very, very close to being saturated.
Of course, that would depend on its temperature, but assuming that you're at