Stoichiometry is the branch of chemistry that uses balanced chemical equations to calculate the amounts of reactants consumed and products formed. The central unit is the mole: 1 mol = 6.022 × 10²³ particles (Avogadro's number). Converting between grams and moles requires the molar mass (grams per mole), obtained by summing atomic masses.
Converting between moles of different substances requires the mole ratio from the balanced equation.
A typical stoichiometry problem follows a roadmap: grams of substance A → moles of A (divide by molar mass) → moles of B (multiply by mole ratio from equation) → grams of B (multiply by molar mass of B). Limiting reagent problems add a step: compute predicted moles of product from each reactant separately, then use the smaller value. The actual yield divided by the theoretical yield gives percent yield.
Understanding Stoichiometry Roadmap
A balanced equation is more than a list of substances. Its coefficients describe fixed particle groupings that must react together. For example, if an equation requires two units of one reactant for every one unit of another, doubling only the first reactant does not double the product.
The second reactant may run out. This comes from conservation of atoms. Chemical reactions rearrange atoms into new substances, but they do not create or destroy the atoms of each element.
Balancing is therefore the foundation of every later calculation. Never change subscripts in a chemical formula to make an equation balance. A subscript changes the substance itself, while a coefficient changes only the amount present.
Units act like signposts through a calculation. Write a unit beside every number, then cancel units as each conversion is made. If a mass is divided by grams per mole, grams cancel and moles remain.
If moles are multiplied by a conversion based on equation coefficients, the original substance cancels and moles of the new substance remain. This method exposes many mistakes before a final answer is reached.
It is especially useful when a problem starts with particles, gas volume, solution concentration, or a mixture of these. The path may look different, but the reaction relationship is used only when the amount has been expressed in moles.
Gas questions need careful attention to conditions. Gas volume does not depend only on how much gas is present. Temperature and pressure affect the space the particles occupy.
A stated molar volume can be used only at the temperature and pressure for which it applies. When conditions change, students often need the ideal gas law, written in words as pressure times volume equals amount in moles times the gas constant times temperature.
Temperature must be in kelvin for this calculation. In laboratory work, gases collected over water need another correction because water vapor contributes part of the measured pressure.
Limiting reactant work is really a comparison of possible outcomes. Treat each available reactant as though the other reactants were unlimited, and calculate the product each could make. The smaller product amount is the one the experiment can actually reach.
Any other reactant is left over, called excess reactant. This idea appears outside textbook problems in manufacturing, cooking, batteries, and environmental chemistry, where one resource controls the final output. Keep extra digits during intermediate steps, then round only at the end using the least precise measurement.
Check that the answer is physically sensible. A product mass can be larger than the mass of one reactant when another reactant supplies atoms, but the total mass is still conserved in a closed system.
Key Facts
- 1 mol = 6.022 × 10²³ particles (Avogadro's number)
- Molar mass (g/mol) = of atomic masses of all atoms in the formula
- Mole ratio from balanced equation connects moles of one substance to another
- Stoichiometry map: grams A → moles A → moles B → grams B
- Limiting reagent: the reactant that runs out first and limits the amount of product
- Percent yield = (actual yield / theoretical yield) × 100%
Vocabulary
- Mole
- The SI unit for amount of substance; 1 mol = 6.022 × 10²³ representative particles (atoms, ions, or molecules).
- Molar mass
- The mass of one mole of a substance in grams per mole; numerically equal to the formula mass in atomic mass units.
- Limiting reagent
- The reactant that is completely consumed first in a reaction, limiting the amount of product that can form.
- Theoretical yield
- The maximum amount of product that can form from the given amounts of reactants, calculated from stoichiometry.
- Percent yield
- The ratio of actual yield to theoretical yield, expressed as a percentage; measures efficiency of a reaction.
Common Mistakes to Avoid
- Skipping the mole ratio step and directly converting grams of reactant to grams of product by mass ratio. The mass ratio between reactant and product is not the same as the mole ratio - you must go through moles and use the balanced equation.
- Using an unbalanced equation for stoichiometry. Mole ratios only have meaning from a balanced equation. Always balance before calculating.
- Forgetting to identify the limiting reagent in problems with two reactants. The reagent present in excess does not determine how much product forms.
- Computing percent yield greater than 100% without questioning it. A percent yield above 100% indicates experimental error - typically incomplete drying, impurities, or a measurement mistake.
Practice Questions
- 1 How many grams of H₂O form when 4.0 g of H₂ reacts with excess O₂? (Balanced: 2H₂ + O₂ → 2H₂O)
- 2 In the reaction N₂ + 3H₂ → 2NH₃, if 14 g of N₂ and 6 g of H₂ are mixed, which is the limiting reagent and how many grams of NH₃ are produced?
- 3 A student synthesizes 4.2 g of aspirin (theoretical yield 5.0 g). What is the percent yield?