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5.3: Chemical Formulas- How to Represent Compounds

CHEM 114: Introductory Chemistry · LibreTexts

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    From LibreTexts, under CC BY-NC-SA 4.0, as the source states it; its equations are written in TeX, which the PDF shows typeset. This page at LibreTexts.

    Chapter 1
    1. Chapter 1: The Chemical World
    2. 1.1: The Scope of Chemistry
    3. 1.2: Chemicals Compose Ordinary Things
    4. 1.3: Hypothesis, Theories, and Laws
    5. 1.4: The Scientific Method: How Chemists Think
    6. 1.5: A Beginning Chemist: How to Succeed
     •  Chapter 2
    1. Chapter 2: Measurement and Problem Solving
    2. 2.1: Taking Measurements
    3. 2.2: Scientific Notation: Writing Large and Small Numbers
    4. 2.3: Significant Figures: Writing Numbers to Reflect Precision
    5. 2.4: Significant Figures in Calculations
    6. 2.5: The Basic Units of Measurement
    7. 2.6: Problem Solving and Unit Conversions
    8. 2.7: Solving Multistep Conversion Problems
    9. 2.8: Units Raised to a Power
    10. 2.9: Density
    11. 2.10: Numerical Problem-Solving Strategies and the Solution Map
    12. 2.E: Measurement and Problem Solving (Exercises)
     •  Chapter 3
    1. Chapter 3: Matter and Energy
    2. 3.1: In Your Room
    3. 3.2: What is Matter?
    4. 3.3: Classifying Matter According to Its State: Solid, Liquid, and Gas
    5. 3.4: Classifying Matter According to Its Composition
    6. 3.5: Differences in Matter: Physical and Chemical Properties
    7. 3.6: Changes in Matter: Physical and Chemical Changes
    8. 3.7: Conservation of Mass: There is No New Matter
    9. 3.8: Energy
    10. 3.9: Energy and Chemical and Physical Change
    11. 3.10: Temperature: Random Motion of Molecules and Atoms
    12. 3.11: Temperature Changes: Heat Capacity
    13. 3.12: Energy and Heat Capacity Calculations
    14. 3.E: Exercises
     •  Chapter 4
    1. Chapter 4: Atoms and Elements
    2. 4.1: Experiencing Atoms at Tiburon
    3. 4.2: Indivisible: The Atomic Theory
    4. 4.3: The Nuclear Atom
    5. 4.4: The Properties of Protons, Neutrons, and Electrons
    6. 4.5: Elements: Defined by Their Numbers of Protons
    7. 4.6: Looking for Patterns: The Periodic Law and the Periodic Table
    8. 4.7: Ions: Losing and Gaining Electrons
    9. 4.8: Isotopes: When the Number of Neutrons Varies
    10. 4.9: Atomic Mass: The Average Mass of an Element’s Atoms
      •  Chapter 5
    1. Chapter 5: Molecules and Compounds
    2. 5.1: Sugar and Salt
    3. 5.2: Compounds Display Constant Composition
    4. 5.3: Chemical Formulas: How to Represent Compounds
    5. 5.4: A Molecular View of Elements and Compounds
    6. 5.5: Writing Formulas for Ionic Compounds
    7. 5.6: Nomenclature: Naming Compounds
    8. 5.7: Naming Ionic Compounds
    9. 5.8: Naming Molecular Compounds
    10. 5.9: Naming Acids
    11. 5.10: Nomenclature Summary
    12. 5.11: Formula Mass: The Mass of a Molecule or Formula Unit
      •  Chapter 6
    1. Chapter 6: Chemical Composition
    2. 6.1: How Much Sodium?
    3. 6.2: Counting Nails by the Pound
    4. 6.3: Counting Atoms by the Gram
    5. 6.4: Counting Molecules by the Gram
    6. 6.5: Chemical Formulas as Conversion Factors
    7. 6.6: Mass Percent Composition of Compounds
    8. 6.7: Mass Percent Composition from a Chemical Formula
    9. 6.8: Calculating Empirical Formulas for Compounds
    10. 6.9: Calculating Molecular Formulas for Compounds
      •  Chapter 7
    1. Chapter 7: Chemical Reactions
    2. 7.1: Grade School Volcanoes, Automobiles, and Laundry Detergents
    3. 7.2: Evidence of a Chemical Reaction
    4. 7.3: The Chemical Equation
    5. 7.4: How to Write Balanced Chemical Equations
    6. 7.5: Aqueous Solutions and Solubility: Compounds Dissolved in Water
    7. 7.6: Precipitation Reactions: Reactions in Aqueous Solution That Form a Solid
    8. 7.7: Writing Chemical Equations for Reactions in Solution: Molecular, Complete Ionic, and Net Ionic Equations
    9. 7.8: Acid–Base and Gas Evolution Reactions
    10. 7.9: Oxidation–Reduction Reactions
    11. 7.10: Classifying Chemical Reactions
    12. 7.11: The Activity Series
     •  Chapter 8
    1. Chapter 8: Quantities in Chemical Reactions
    2. 8.1: Climate Change: Too Much Carbon Dioxide
    3. 8.2: Stoichiometry
    4. 8.3: Making Molecules: Mole-to-Mole Conversions
    5. 8.4: Making Molecules: Mass-to-Mass Conversions
    6. 8.5: Limiting Reactant, Theoretical Yield, and Percent Yield
    7. 8.6: Limiting Reactant, Theoretical Yield, and Percent Yield from Initial Masses of Reactants
    8. 8.7: Enthalpy: A Measure of the Heat Evolved or Absorbed in a Reaction
    Chapter 9
    1. Chapter 9: Electrons in Atoms and the Periodic Table
    2. 9.1: Blimps, Balloons, and Models of the Atom
    3. 9.2: Light: Electromagnetic Radiation
    4. 9.3: The Electromagnetic Spectrum
    5. 9.4: The Bohr Model: Atoms with Orbits
    6. 9.5: The Quantum-Mechanical Model: Atoms with Orbitals
    7. 9.6: Quantum-Mechanical Orbitals and Electron Configurations
    8. 9.7: Electron Configurations and the Periodic Table
    9. 9.8: The Explanatory Power of the Quantum-Mechanical Model
    10. 9.9: Periodic Trends: Atomic Size, Ionization Energy, and Metallic Character
     •  Chapter 10
    1. Chapter 10: Chemical Bonding
    2. 10.1: Bonding Models and AIDS Drugs
    3. 10.2: Representing Valence Electrons with Dots
    4. 10.3: Lewis Structures of Ionic Compounds: Electrons Transferred
    5. 10.4: Covalent Lewis Structures: Electrons Shared
    6. 10.5: Writing Lewis Structures for Covalent Compounds
    7. 10.6: Resonance: Equivalent Lewis Structures for the Same Molecule
    8. 10.7: Predicting the Shapes of Molecules
    9. 10.8: Electronegativity and Polarity: Why Oil and Water Don’t Mix
     •  Chapter 11
    1. Chapter 11: Gases
    2. 11.1: Extra-Long Straws
    3. 11.2: Kinetic Molecular Theory: A Model for Gases
    4. 11.3: Pressure: The Result of Constant Molecular Collisions
    5. 11.4: Boyle’s Law: Pressure and Volume
    6. 11.5: Charles’s Law: Volume and Temperature
    7. 11.6: Gay-Lussac's Law: Temperature and Pressure
    8. 11.7: The Combined Gas Law: Pressure, Volume, and Temperature
    9. 11.8: Avogadro’s Law: Volume and Moles
    10. 11.9: The Ideal Gas Law: Pressure, Volume, Temperature, and Moles
    11. 11.10: Mixtures of Gases: Why Deep-Sea Divers Breathe a Mixture of Helium and Oxygen
    12. 11.11: Gases in Chemical Reactions
     •  Chapter 12
    1. Chapter 12: Liquids, Solids, and Intermolecular Forces
    2. 12.1: Interactions between Molecules
    3. 12.2: Properties of Liquids and Solids
    4. 12.3: Intermolecular Forces in Action: Surface Tension and Viscosity
    5. 12.4: Evaporation and Condensation
    6. 12.5: Melting, Freezing, and Sublimation
    7. 12.6: Types of Intermolecular Forces: Dispersion, Dipole–Dipole, Hydrogen Bonding, and Ion-Dipole
    8. 12.7: Types of Crystalline Solids: Molecular, Ionic, and Atomic
    9. 12.8: Water: A Remarkable Molecule
     •  Chapter 13
    1. Chapter 13: Solutions
    2. 13.1: Prelude - Tragedy in Cameroon
    3. 13.2: Solutions: Homogeneous Mixtures
    4. 13.3: Solutions of Solids Dissolved in Water: How to Make Rock Candy
    5. 13.4: Solutions of Gases in Water: How Soda Pop Gets Its Fizz
    6. 13.5: Solution Concentration: Mass Percent
    7. 13.6: Solution Concentration: Molarity
    8. 13.7: Solution Dilution
    9. 13.8: Solution Stoichiometry
    10. 13.9: Freezing Point Depression and Boiling Point Elevation: Making Water Freeze Colder and Boil Hotter
    11. 13.10: Osmosis: Why Drinking Salt Water Causes Dehydration
      •  Chapter 14
    1. Chapter 14: Acids and Bases
    2. 14.1: Sour Patch Kids and International Spy Movies
    3. 14.2: Acids: Properties and Examples
    4. 14.3: Bases: Properties and Examples
    5. 14.4: Molecular Definitions of Acids and Bases
    6. 14.5: Reactions of Acids and Bases
    7. 14.6: Acid–Base Titration: A Way to Quantify the Amount of Acid or Base in a Solution
    8. 14.7: Strong and Weak Acids and Bases
    9. 14.8: Water: Acid and Base in One
    10. 14.9: The pH and pOH Scales: Ways to Express Acidity and Basicity
    11. 14.10: Buffers: Solutions That Resist pH Change
     

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    Learning Objectives
    • Determine the number of different atoms in a formula.
    • Define chemical formula, molecular formula, and empirical formula.

    A chemical formula is an expression that shows the elements in a compound and the relative proportions of those elements. Water is composed of hydrogen and oxygen in a 2:1 ratio. The chemical formula for water is \(\ce{H_2O}\). Sulfuric acid is one of the most widely produced chemicals in the United States and is composed of the elements hydrogen, sulfur, and oxygen. The chemical formula for sulfuric acid is \(\ce{H_2SO_4}\).

    Certain groups of atoms are bonded together to form what is called a polyatomic ion that acts as a single unit. Polyatomic ions are discussed in more detail in Section 5.5. Polyatomic ions are enclosed in parenthesis followed by a subscript if more than one of the same ion exist in a chemical formula. The formula \(\ce{Ca3(PO4)2}\) represents a compound with the following:

    3 Ca atoms + 2 PO43- ions

    To count the total number of atoms for formulas with polyatomic ions enclosed in parenthesis, use the subscript as a multiplier for each atom or number of atoms.

    Ca3(PO4)2

    3 Ca + 2 x1 P + 2 x 4 O = 3 Ca atoms + 2 P atoms + 8 O atoms

    Molecular Formula

    A molecular formula is a chemical formula of a molecular compound that shows the kinds and numbers of atoms present in a molecule of the compound. Ammonia is a compound of nitrogen and hydrogen as shown below:

    Diagram explaining ammonia (NH₃) structure; notes on nitrogen's lack of subscript and presence of three hydrogen atoms.
    Figure \(\PageIndex{1}\): The molecular formula for ammonia. NH3. There is one atom of nitrogen and 3 atoms of hydrogen in a molecule of ammonia.

    Note from the example that there are some standard rules to follow in writing molecular formulas. The arrangements of the elements depend on the particular structure, which is not of concern at this point. The number of atoms of each kind is indicated by a subscript following the atom. If there is only one atom, no number is written. If there is more than one atom of a specific kind, the number is written as a subscript following the atom. We would not write \(\ce{N_3H}\) for ammonia, because that would mean that there are three nitrogen atoms and one hydrogen atom in the molecule, which is incorrect.

    Empirical Formula

    An empirical formula is a formula that shows the elements in a compound in their lowest whole-number ratio. Glucose is an important simple sugar that cells use as their primary source of energy. Its molecular formula is \(\ce{C_6H_{12}O_6}\). Since each of the subscripts is divisible by 6, the empirical formula for glucose is \(\ce{CH_2O}\). When chemists analyze an unknown compound, often the first step is to determine its empirical formula.

    • molecular formula: \(\ce{C_6H_{12}O_6}\)
    • empirical formula: \(\ce{CH_2O}\)

    There are a great many compounds whose molecular and empirical formulas are the same. If the molecular formula cannot be simplified into a smaller whole-number ratio, as in the case of \(\ce{H_2O}\) or \(\ce{P_2O_5}\), then the empirical formula is also the molecular formula.

    Summary

    • A chemical formula is an expression that shows the elements in a compound and the relative proportions of those elements.
    • If only one atom of a specific type is present, no subscript is used.
    • For atoms that have two or more of a specific type of atom present, a subscript is written after the symbol for that atom.
    • Polyatomic ions in chemical formulas are enclosed in parentheses followed by a subscript if more than one of the same type of polyatomic ion exist.
    • Molecular formulas do not indicate how the atoms are arranged in the molecule.
    • The empirical formula tells the lowest whole-number ratio of elements in a compound. The empirical formula does not show the actual number of atoms.

    5.3: Chemical Formulas- How to Represent Compounds is shared under a CC BY-NC-SA 4.0 license and was authored, remixed, and/or curated by LibreTexts.