1 mmol --------------------- 1000 <span>µmol
( mmol ) -------------------- </span> 38231 µmol
mmol = 38231*1 / 1000
mmol = 38231/ 1000
=> 38.231 mmol
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 = <span>18.3%</span>