In a solution of KBr and water; KBr is the solute and water is the solvent;
Therefore; to achieve 3% by mass; it means we are going to have 3% of the mass being the solute and the other 97 % being the solvent.
Thus; KBr (solute) = 3/100 × 300 (total mass) = 9 g
Hence; the appropriate masses will be; 9.00 g of KBr and 291 g of water.
