Answer: mass fraction of the solution is 4.8%.
Explanation: To find the mass fraction of a solution, we need to divide the mass of the solute (what's being dissolved) by the total mass of the solution (solute + solvent), and then multiply by 100 to express the result as a percentage.
In this case, the solute is 6.3 g NaCl and the solvent is 125.0 g H2O. Therefore, the total mass of the solution is:
6.3 g NaCl + 125.0 g H2O = 131.3 g solution
Now we can calculate the mass fraction:
mass fraction = (mass of NaCl / total mass of solution) x 100%
mass fraction = (6.3 g / 131.3 g) x 100%
mass fraction = 0.048 x 100%
mass fraction = 4.8%
Therefore, the mass fraction of the solution is 4.8%.