How can the student make a quantitative observation of the CO2 collected in the balloons?

The student can measure the volume of CO2 collected in each balloon using a graduated cylinder or syringe to calculate the total volume of gas. They could then compare the mass of the balloons before and after the experiment to determine the mass of CO2 collected. Finally, they could use a color-changing pH indicator to analyze the acidity of the collected gas, indicating the presence of CO2.