Well this may be difficult unless you have some good lab equip!. The problem is this: when the chemical reaction runs the TOTAL MASS will be the same of the isolated reactants baking soda plus vinegar as the products of CO₂ gas plus a solution with stuff dissolved in it (i.e. the Na aqueous species).

The issue is in weighing the before (easy) with the after (hard) because the CO₂ is a gas and you can't let any of that escape and it is hard to capture and weigh properly.

One suggestion is to watch the reaction take place in a closed container on a scale. Consider putting the vinegar and baking soda in separate partitions in the container, sealing the container, checking the mass, then tipping the container to mix the reactants. The final mass should be unchanged.

Why seal the container? Mass conservation is easy to show when all the products are solids or liquids, but for this reaction we would like to account for the carbon dioxide produced as well. Fortunately we don't expect buoyancy to play a role since CO2 is still 1.7x denser than air, so the scale will probably do a good job. (It's still worth testing, though!)

You can show this with the following:

1. Repeat the experiment in a number of different containers (glass, ceramic, metal, plastic). This will tell the students that the container isn't part of the reaction, since you can do it in different containers.

2. Do the reaction inside of a plastic bag. This will fill up the bag as the carbon dioxide gas is released.