Molar mass of MgCO3 is 84.313 g/mol You can calculate this from data on the periodic table: Molar mass Mg = 24.305g/mol molar mass C = 12.011g/mol molar mass O = 15.999g/mol mass 3 mol = 47.997g Total = 84.313g/mol
Mass to be used in 1.2L of 1.5M solution = 84.313g * 1.2L * 1.5mol /L = 151.763g I have not taken significant figures into account The balanced equation you provide is not necessary in this calculation