Final answer:
When baking soda and vinegar react, carbon dioxide gas is produced, causing the mass of the reactants to decrease because the gas leaves the container.
Step-by-step explanation:
If 25g of baking soda reacts with 25ml of vinegar, the weight after the reaction may not equal 50g. In a chemical reaction, such as the one between baking soda (sodium bicarbonate) and vinegar (acetic acid), a gas is produced. In this case, carbon dioxide (CO2) is produced and will leave the container, causing the mass of the container and contents to decrease. This is because the mass of the gaseous products is not contained in the same way as the reactants. Thus, the answer to how much the mixture should weigh when it's finished would most closely align with option (b) The mass of the container and contents would decrease.