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Chemistry LibreTexts

3.S: Stoichiometry (Summary)

  • Page ID
    • atoms are neither created nor destroyed during any chemical reaction
    • stoichiometry – quantitative nature of chemical formulas and chemical reactions

    3.1: Chemical Equations

    A chemical reaction is described by a chemical equation that gives the identities and quantities of the reactants and the products. In a chemical reaction, one or more substances are transformed to new substances. A chemical reaction is described by a chemical equation, an expression that gives the identities and quantities of the substances involved in a reaction. A chemical equation shows the starting compound(s)—the reactants—on the left and the final compound(s)—the products—on the right.

    • chemical equations – the way chemical reactions are represented
    • reactants – starting substances
    • products – substances produced from a reaction
    • balanced equation – equation with equal atoms on both sides of the equation
    • subscripts should never be changed in balancing an equation
    • coefficients changes only the amount and not identity of the substance

    3.2: Some Simple Patterns of Chemical Reactivity

    By recognizing general patterns of chemical reactivity, you will be able to successfully predict the products formed by a given combination of reactants We can often predict a reaction if we have seen a similar reaction before.

    3.2.1 Using the Periodic Table

    • periodic table can be used to determine reactivity of substances
    • all alkali metals react with water to form their hydroxide compounds and hydrogen

    3.2.2 Combustion in Air

      • rapid reaction that produces a flame
      • most combustion reactions in air involve oxygen
      • hydrocarbons and related compounds produce CO2 and H2O during combustion

    3.2.3 Combination and Decomposition Reactions

      • combination reactions two or more substances react to form one product
      • decomposition reaction one substance produces two or more substances

    3.3: Formula Masses

    The empirical formula of a substance can be calculated from its percent composition, and the molecular formula can be determined from the empirical formula and the compound’s molar mass. The empirical formula of a substance can be calculated from the experimentally determined percent composition, the percentage of each element present in a pure substance by mass. In many cases, these percentages can be determined by combustion analysis.

    3.3.1 The Atomic Mass Scale

    • atomic mass unit (amu) – unit in measuring mass of atoms
    • 1 amu = 1.66054*10-24g and 1 amu = 6.02214*1024amu

    3.3.2 Average Atomic Masses

    • atomic weight – average atomic mass

    3.3.3 Formula and Molecular Weights

    • formula weight – sum of the atomic weights of each atom in its chemical formula
    • molecular weight – same as formula weight

    3.3.4 Percentage composition from Formulas

      • ((atoms of element)(AW)/(FW of compound) * 100

    3.3.5 The Mole

    • avogadro’s number – 6.02*1023 atoms
    • molar mass – numerically equal to its formula weight
    • grams <use molar mass> moles <use avogadro’s number> molecules

    3.5: Empirical Formulas from Analysis

    Molecular formulas tell you how many atoms of each element are in a compound, and empirical formulas tell you the simplest or most reduced ratio of elements in a compound. If a compound's molecular formula cannot be reduced any more, then the empirical formula is the same as the molecular formula. Combustion analysis can determine the empirical formula of a compound, but cannot determine the molecular formula (other techniques can though).

    • empirical formula gives relative number of atoms in each element
    • mass % elements >>> assume 100g sample >>> grams of each element >>> use atomic weights >>> moles of each element >>> calculate mole ratio >>> empirical formula
    • "percent to mass, mass to mol, divide by small, multiply ‘til whole/"

    3.5.1 Molecular Formula from Empirical Formula

      • the subscripts in the molecular formula of a substance are always a whole-number multiple of the corresponding subscripts in its empirical formula

    3.5.2 Combustion Analysis

    3.6: Quantitative Information from Balanced Equations

    Either the masses or the volumes of solutions of reactants and products can be used to determine the amounts of other species in the balanced chemical equation. Quantitative calculations that involve the stoichiometry of reactions in solution use volumes of solutions of known concentration instead of masses of reactants or products. The coefficients in the balanced chemical equation tell how many moles of reactants are needed and how many moles of product can be produced.

    • the coefficients in a balanced chemical equation can be interpreted both as the relative numbers of molecules involved in the reaction and as the relative numbers of moles
    • stoichiometrically equivalent quantities
    • grams reactant >> moles reactant >> moles product >> grams product
    • grams of substance A >> use molar mass of A >> moles of substance A >> use coefficients of A and B from balanced equation >> moles of substance B >> use molar mass of B >> grams of substance B

    3.7: Limiting Reactants

    The stoichiometry of a balanced chemical equation identifies the maximum amount of product that can be obtained. The stoichiometry of a reaction describes the relative amounts of reactants and products in a balanced chemical equation. A stoichiometric quantity of a reactant is the amount necessary to react completely with the other reactant(s). If a reactant remains unconsumed after complete reaction has occurred, it is in excess. The reactant that is consumed first is the limiting reagent.

    • limiting reactant – limits the amount of product formed

    3.7.1 Theoretical Yields

    • theoretical yield – the amount of product that is calculated to form
    • actual yield – the amount of product actually formed

    \[ \text{percent yield} = \dfrac{\text{actual yield}}{\text{theoretical yield}} \times 100\% \nonumber\]