To 200 g of an 18% sulfuric acid solution was added 150 g of water. The mass fraction of acid in the new solution is …

The mass fraction of a substance in a solution is defined as the ratio of the mass of the solute to the mass of the solution
w (H2SO4) = m (H2SO4) / m solution. Hence, the mass of the solute is m (H2SO4) = w (H2SO4) * m solution.
We convert the mass fraction of sulfuric acid into fractions, dividing 18% by 100% w (H2SO4) = 18% / 100% = 0.18. Now we find the mass of sulfuric acid in the solution m (H2SO4) = 0.18 * 200g = 36g. After adding 150 g of water, the mass of the solution changed and became equal to m solution = 200 g + 150 g = 350 g. The mass of sulfuric acid in the new solution did not change. Calculate the mass fraction of sulfuric acid in the new solution w (H2SO4) = 36g / 350g = 0.1029 or 10.29%