Answer:
5.0% by mass
Step-by-step explanation:
Percent by mass is 100 times the ratio between the mass of solute (Magnesium citrate) in the mass of solution.
Mass of Magnesium citrate -Molar mass: 214.41g/mol-:
0.1mol * (214.41g/mol) = 21.441g
Assuming the volume of the solution is just due the solvent -Water-, the mass of water is 428g.
Percent by mass is:
21.441g / 428g * 100 =
5.0% by mass