Stoichiometry: Calculations with Chemical Formulas and Equations

Balancing Chemical Equations

Balancing chemical equations ensures that the same number of each type of atom appears on both sides of the equation, reflecting the law of conservation of mass.

• Diatomic Elements: Elements that naturally exist as molecules of two atoms: H2, O2, F2, Cl2, Br2, I2.

• Writing Chemical Formulas: Use the charge criss-cross method to write formulas for ionic compounds. For example, magnesium nitride is written as Mg3N2.

• Balancing Steps:

  • Adjust coefficients (numbers in front of formulas), not subscripts.

  • Check that the number of each atom is equal on both sides.

Additional info: Never change the chemical identity of substances by altering subscripts; only coefficients are changed to balance equations.

Simple Patterns of Chemical Reactivity

Chemical reactions can be classified into several types based on the rearrangement of atoms and molecules.

• Combination (Synthesis) Reactions: Two or more substances combine to form a single product. General form:

• Decomposition Reactions: A single compound breaks down into two or more simpler substances. General form:

• Combustion Reactions: A substance reacts with oxygen, releasing energy (often as a flame), and typically forms CO2 and H2O. Example:

Formula Weights and Percent Composition

The formula weight (or molecular weight for covalent compounds) is the sum of the atomic weights of all atoms in a chemical formula.

• Formula Weight: Add the atomic weights of all atoms in the formula. Example: For Ca(NO2)3, the formula weight is 164.1 amu.

• Smallest Units: Molecular compounds consist of molecules; ionic compounds consist of formula units.

• Percent Composition: The percentage by mass of each element in a compound. Formula:

Avogadro’s Number and the Mole

The mole is a fundamental unit in chemistry that represents a specific number of particles (atoms, molecules, or ions).

• Avogadro’s Number: particles per mole.

• Mole Definition: The amount of substance whose mass in grams equals its atomic or formula weight.

• Molar Mass: The mass of one mole of a substance (g/mol), numerically equal to the formula weight in amu.

Interconversions Between Grams, Moles, and Particles

Conversions between mass, moles, and number of particles are fundamental in stoichiometry. You must always convert through moles:

• Grams → Moles: Divide by molar mass.

• Moles → Particles: Multiply by Avogadro’s number.

• Particles → Moles: Divide by Avogadro’s number.

• Moles → Grams: Multiply by molar mass.

Additional info: You cannot convert directly between grams and particles without first converting to moles.

Empirical Formula and Its Determination

The empirical formula shows the simplest whole-number ratio of atoms in a compound. It can be determined from percent composition or combustion data.

• From Percent Composition:

  1. Assume 100 g of compound (percentages become grams).

  2. Convert grams of each element to moles (moles = grams / atomic mass).

  3. Divide each by the smallest number of moles to get the ratio (subscripts).

• From Combustion Data:

  1. Find grams of C from CO2 produced (moles CO2 = moles C).

  2. Find grams of H from H2O produced (moles H2O × 2 = moles H).

  3. Find grams of O by subtracting grams of C and H from total mass.

Quantitative Information from Balanced Equations

Balanced chemical equations provide the mole ratios of reactants and products, which are essential for stoichiometric calculations.

• Coefficients: Indicate the number of moles (or molecules) of each substance involved.

• Mass Conservation: The total mass of reactants equals the total mass of products, but the number of moles may differ.

• Stoichiometric Calculations: Use the coefficients to relate moles of one substance to moles of another.

Stoichiometry Calculation Steps

Additional info: Never use the molar mass of one substance with the grams of another; always match the substance in both mass and molar mass.

Limiting Reactants and Percent Yield

In chemical reactions, the limiting reactant is the substance that is completely consumed first, thus limiting the amount of product formed.

• Limiting Reactant: The reactant that runs out first and determines the maximum amount of product.

• Excess Reactant: The reactant that remains after the reaction is complete.

• Theoretical Yield: The maximum amount of product predicted by stoichiometry, based on the limiting reactant.

• Actual Yield: The amount of product actually obtained from the reaction.

• Percent Yield: The ratio of actual yield to theoretical yield, expressed as a percentage. Formula:

Example: If you have 8 frames and 18 wheels to make tricycles (each requiring 1 frame and 3 wheels), you can make only 6 tricycles before running out of wheels. Wheels are the limiting reactant; frames are in excess.

Summary Table: Key Stoichiometry Concepts