Question

If 45.8 grams of MgO are dissolved in 205 grams of water, what is the concentration of the solution in percent by mass

Answer 1

Although it's true that MgO is only very slightly soluble in water, you can consider the mixture a conventional solution for the purposes of this question. The % mass is the mass of MgO divided by the mass of the entire solution:

45.8 / (45.8 + 205) * 100 = 18.3%