Stoichiometry
Stoichiometry is the branch of chemistry that deals with the quantitative relationships between reactants and products in chemical reactions. It uses balanced chemical equations to calculate the masses, volumes, or moles of substances involved in a reaction. Stoichiometry is fundamental to industrial chemistry, pharmaceutical manufacturing, and any process requiring precise control of chemical quantities.
Key Formula
moles of product = moles of reactant × (coefficient of product / coefficient of reactant)
LaTeX: \text{moles of product} = \text{moles of reactant} \times \dfrac{\text{stoichiometric coefficient of product}}{\text{stoichiometric coefficient of reactant}}
| Symbol | Meaning | Unit |
|---|---|---|
| n_product | Moles of desired product | mol |
| n_reactant | Moles of known reactant | mol |
| coeff | Stoichiometric coefficients from balanced equation | dimensionless |
Worked Example
Problem
In the reaction 2H₂ + O₂ → 2H₂O, how many moles of water are produced when 4 mol of H₂ reacts completely?
Solution
Step 1: Write the balanced equation: 2H₂ + O₂ → 2H₂O. Step 2: Identify the mole ratio of H₂ to H₂O from the coefficients: 2 mol H₂ produces 2 mol H₂O, so ratio = 2/2 = 1. Step 3: Apply the mole ratio: moles H₂O = 4 mol H₂ × (2 mol H₂O / 2 mol H₂) = 4 mol H₂O.
Answer
4 mol of H₂O
Steps in a Stoichiometric Calculation
| Step | Action | Tool Used | Example |
|---|---|---|---|
| 1 | Write balanced equation | Chemical equation balancer | 2H₂ + O₂ → 2H₂O |
| 2 | Convert mass to moles | Molar mass | 4 g H₂ ÷ 2 g/mol = 2 mol |
| 3 | Apply mole ratio | Stoichiometric coefficients | 2 mol H₂ → 2 mol H₂O |
| 4 | Convert moles to mass | Molar mass | 2 mol H₂O × 18 g/mol = 36 g |
| 5 | Check units and sig figs | Dimensional analysis | g, mol, L as required |
Interactive Tools
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Related Terms
Mole (chemistry)
The mole is the SI base unit for the amount of substance, defined as exactly 6.02214076 × 10²³ elementary entities (atoms, molecules, ions, or other particles). It provides chemists with a practical bridge between the atomic scale and macroscopic, measurable quantities in the laboratory. One mole of any substance contains the same number of particles, making it the universal counting unit of chemistry.
Molar Mass
Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol), and numerically equal to the substance's relative atomic or molecular mass in unified atomic mass units. It is calculated by summing the atomic masses of all atoms in one formula unit of the substance, using values from the periodic table. Molar mass is the essential conversion factor between grams (measurable in the lab) and moles (used in chemical calculations).
Percent Yield
Percent yield is the ratio of the actual yield of a chemical reaction (the amount of product experimentally obtained) to the theoretical yield (the maximum amount predicted by stoichiometry), expressed as a percentage. Values below 100% indicate losses due to incomplete reactions, side reactions, product loss during purification, or measurement errors. Percent yield is a key quality metric in synthetic chemistry, pharmaceuticals, and industrial chemical production.
From Greek "stoicheion" (element) and "metron" (measure). The term was coined by German chemist Jeremias Benjamin Richter in 1792 to describe the law of definite proportions in chemical reactions.