Answer:
Some of the product is a gas that goes to the atmosphere.
Explanation:
Based on the chemical reaction of baking soda (Sodium bicarbonate: NaHCO₃) with vinegar (Acetic acid: CH₃COOH):
NaHCO₃(aq) + CH₃COOH(aq) → H₂O(aq) + CO₂(g) + CH₃COONa(aq)
When the reaction occurs, one of the products (CO₂) is a gas that will disappear of the mixture to stay in the atmosphere. That is the reason why there are missing 3 grams.