Stoichiometry

Stoichiometry is the calculation of the relative quantities of reactants and products in chemical reactions. It is based on the Law of Conservation of Mass, which states that matter cannot be created or destroyed in a chemical reaction.

🔢 Key Principles:

1. Balanced Chemical Equations

   • A reaction must be balanced before stoichiometric calculations can be done.

   • Example:

     2H2+O2→2H2O2H_2 + O_2 \rightarrow 2H_2O

     This tells us that 2 moles of hydrogen gas react with 1 mole of oxygen gas to form 2 moles of water.

2. Mole Ratios

   • Derived from the coefficients in a balanced equation.

   • Used to convert between amounts of different substances.

3. Conversions Involved:

   • Grams ↔ Moles using molar mass

   • Moles ↔ Particles using Avogadro’s number

   • Moles ↔ Volume (for gases at STP, 1 mol = 22.4 L)

🧪 Example Problem:

How many grams of water are produced when 4 grams of hydrogen gas react with excess oxygen?

1. Balanced equation:

   2H2+O2→2H2O2H_2 + O_2 \rightarrow 2H_2O

2. Moles of H2H_2:

   4 g2.02 g/mol≈1.98 mol\frac{4\,g}{2.02\,g/mol} \approx 1.98\,mol

3. Mole ratio H2:H2O=2:2H_2 : H_2O = 2:2, so

   1.98 mol H2→1.98 mol H2O1.98\,mol\,H_2 \rightarrow 1.98\,mol\,H_2O

4. Mass of water:

   1.98 mol×18.02 g/mol≈35.7 g1.98\,mol \times 18.02\,g/mol \approx 35.7\,g

✔️ Answer: ~35.7 grams of water produced

🧠 Applications of Stoichiometry:

• Determining limiting reactants and excess reactants

• Calculating yields (theoretical and percent)

• Designing industrial chemical processes efficiently

• Environmental chemistry (e.g., predicting CO₂ emissions)