Final answer:
The mass percent of NaCl in a solution with 15 g of NaCl dissolved in 45 g of H₂O is calculated by dividing the mass of NaCl by the total mass of the solution and then multiplying by 100, which results in 25%.
Step-by-step explanation:
The mass percent of a solution is calculated as the mass of the solute divided by the total mass of the solution, multiplied by 100. To find the mass percent of NaCl in a solution prepared from 15 g of NaCl dissolved in 45 g of water (H₂O), we first need to determine the total mass of the solution. The total mass is the sum of the mass of the NaCl and the mass of the water, which is 15 g + 45 g = 60 g.
Next, we calculate the mass percent of NaCl: (mass of NaCl / total mass of solution) x 100% = (15 g / 60 g) x 100% = 25%. Thus, the mass/mass percent concentration of NaCl in the solution is 25%.