(Originais Teachy 2023) - Question Very Hard of Chemistry

To determine the efficiency of a water purification process, a technician analyzes the concentration of a pollutant in a sample. The concentration of the pollutant is 2.4 parts per million (ppm) in a volume of 250 mL. However, a second technician measures the concentration of the same pollutant in another sample and obtains 3.5 parts per billion (ppb) in a volume of 500 L. Considering the difference in concentration units and volume, explain, based on stoichiometric calculation principles, how would you convert the measured concentrations to a single unit, such as mass percentage (m/m). Additionally, indicate which sample presents the highest concentration of the pollutant in terms of mass percentage and justify your answer with the necessary calculations.