In chemistry, stoichiometry is primarily based on which fundamental scientific law that relates the masses of reactants and products in a chemical reaction?

Introduction / Context:
Stoichiometry is a core topic in introductory and advanced chemistry, and it deals with the quantitative relationships between reactants and products in chemical reactions. Whenever we calculate how many grams of a substance are needed or produced, we are using stoichiometric ideas. To understand stoichiometry correctly, it is essential to know the basic law on which all these mass relationships are based. This law ensures that our calculations match what actually happens in a real chemical reaction, whether in a school laboratory or in a large industrial plant.

Given Data / Assumptions:
- The question asks which fundamental scientific law forms the basis of stoichiometry. - Stoichiometry deals with the mass relationships of reactants and products during chemical reactions. - We assume reactions occur in a closed system without gain or loss of matter to the surroundings.

Concept / Approach:
Stoichiometry assumes that the total mass of substances present before a chemical reaction is exactly equal to the total mass after the reaction. This idea comes directly from the law of conservation of mass. The law states that matter cannot be created or destroyed in a chemical reaction, it can only change form and combine in different ways. Therefore, when we balance chemical equations, we make sure that the number of atoms of each element is the same on both sides, which corresponds to equal mass before and after the reaction.

Step-by-Step Solution:
Step 1: Recall the definition of stoichiometry as the quantitative study of reactants and products in a chemical reaction. Step 2: Recognize that stoichiometric calculations rely on balanced chemical equations where the number of atoms of each element is the same on both sides. Step 3: Understand that equal numbers of atoms on both sides imply that the total mass remains constant during the reaction. Step 4: Connect this idea to the law of conservation of mass, which states that total mass is conserved in every chemical reaction occurring in a closed system. Step 5: Conclude that stoichiometry is fundamentally based on the law of conservation of mass and not on the other listed physical laws.

Verification / Alternative check:
As a quick check, think about any balanced chemical equation you have studied. For example, in the reaction of hydrogen with oxygen to form water, the total mass of hydrogen and oxygen before the reaction is equal to the total mass of water produced. This confirms that stoichiometric calculations assume mass conservation. Energy may also be conserved, but stoichiometry directly uses mass relationships rather than energy values. This supports the choice of the law of conservation of mass as the correct answer.

Why Other Options Are Wrong:
Option A (Law of conservation of energy) is about the conservation of energy, not mass. While important in thermodynamics, it is not the primary basis for stoichiometric mass calculations. Option C (Newton first law of motion) deals with inertia and motion of bodies and is part of classical mechanics, not directly related to mass balancing in chemical reactions. Option D (Kepler planetary laws) describes the motion of planets around the sun and has no connection to chemical reaction mass relationships.

Common Pitfalls:
Students sometimes confuse conservation of mass with conservation of energy because both are fundamental conservation laws. Another common mistake is thinking that mass can disappear when gases are formed, for example when bubbles leave a solution. In reality, if the system is considered as a whole and closed, the total mass remains constant. Forgetting to balance chemical equations properly also leads to errors in stoichiometric calculations. Always check that the equation is balanced before doing any mass or mole calculations.

Final Answer:
The correct answer is: Law of conservation of mass.