Stoichiometry is based on the law?

Step 1: Analyze the question Stoichiometry is the calculation of the quantities of reactants and products in a chemical reaction. The question asks which law is the foundation of these calculations.

Step 2: Explain the concept When we balance a chemical equation, we make sure the number of atoms of each element is the same on both sides. This works because of the Law of Conservation of Mass, which says that matter is neither created nor destroyed in a chemical reaction. The total mass of the reactants equals the total mass of the products.

Step 3: Show the solution Take the formation of water:

2H₂ + O₂ → 2H₂O

• Left side: 4 H atoms and 2 O atoms, so the mass is (4 × 1) + (2 × 16) = 36 g
• Right side: 2 × (2 + 16) = 36 g

The masses are equal, so we can use mole ratios (2 : 1 : 2) to find how much reactant is needed or how much product forms. That is stoichiometry.

Step 4: Check the other options

• A. Multiple proportions: This describes how elements combine in different whole-number ratios to form different compounds (e.g. CO and CO₂). It is not the basis of stoichiometry.
• B. Constant composition: A compound always contains the same elements in the same proportion by mass. It supports stoichiometry, but the main basis for balancing and mass calculations is conservation of mass.
• C. Conservation of energy: This is about energy, not the amounts of substances.
• D. Conservation of mass: This is the basis of balanced equations and mole calculations. ✔

Answer: D. conservation of mass

Tip: If a question mentions balancing equations or "mass of reactants = mass of products," think conservation of mass. For "same compound, same ratio always," think constant composition. For "different compounds, simple whole-number ratios," think multiple proportions.