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Intro to Chemistry
Table of Contents
Unit 3 Overview: Substances and Solutions Composition
3.1 Formula Mass and the Mole Concept
3.2 Determining Empirical and Molecular Formulas
3.3 Molarity
3.4 Other Units for Solution Concentrations

💏intro to chemistry review

3.2 Determining Empirical and Molecular Formulas

Citation:

Chemical formulas are like a secret code for molecules. They tell us what elements make up a compound and how many atoms of each are present. Empirical formulas show the simplest ratio, while molecular formulas reveal the actual number of atoms.

Understanding these formulas is crucial for chemists. They help us calculate quantities in reactions, predict properties, and identify unknown substances. Mastering how to derive and interpret these formulas opens up a world of chemical knowledge and problem-solving skills.

Empirical and Molecular Formulas

Percent composition of compounds

  • Represents the mass percentage of each element in a compound calculated by dividing the mass of each element by the total mass of the compound and multiplying by 100
  • Formula for percent composition: $\text{Percent composition of element} = \frac{\text{Mass of element}}{\text{Total mass of compound}} \times 100%$
  • Steps to calculate percent composition:
    1. Determine the mass of each element in the compound using atomic masses (e.g., C = 12.01 g/mol, H = 1.01 g/mol, O = 16.00 g/mol)
    2. Calculate the total mass of the compound by adding the masses of all elements
    3. Divide the mass of each element by the total mass and multiply by 100 to obtain percent composition (e.g., glucose (C₆H₁₂O₆): C = 40.00%, H = 6.72%, O = 53.28%)

Empirical formulas from elemental data

  • Represents the simplest whole-number ratio of atoms in a compound derived from elemental composition data
  • Steps to derive empirical formula:
    1. Convert the percent composition or mass of each element to moles using the molar mass with the formula: $\text{Moles of element} = \frac{\text{Mass of element}}{\text{Molar mass of element}}$
    2. Divide each mole value by the smallest mole value to obtain the simplest whole-number ratio (e.g., C:H:O = 1:2:1 for glucose)
    3. If necessary, multiply the ratio by an integer to obtain whole numbers (e.g., empirical formula of glucose: CH₂O)
  • The empirical formula may or may not be the same as the molecular formula depending on the actual number of atoms in the compound
  • Empirical formulas are essential in stoichiometry calculations and writing balanced chemical equations

Molecular formulas using empirical formulas

  • Represents the actual number of atoms of each element in a compound determined using empirical formulas and molar mass information
  • Steps to determine molecular formula:
    1. Calculate the empirical formula mass by adding the atomic masses of each element in the empirical formula (e.g., CH₂O = 12.01 + 2(1.01) + 16.00 = 30.03 g/mol)
    2. Divide the molar mass of the compound by the empirical formula mass to obtain a ratio using the formula: $\text{Ratio} = \frac{\text{Molar mass of compound}}{\text{Empirical formula mass}}$ (e.g., glucose molar mass = 180.16 g/mol, ratio = 180.16 / 30.03 = 6)
    3. Multiply the subscripts in the empirical formula by the ratio to obtain the molecular formula (e.g., glucose molecular formula: (CH₂O)₆ = C₆H₁₂O₆)
  • The molecular formula will always be a whole-number multiple of the empirical formula (e.g., benzene: empirical formula = CH, molecular formula = C₆H₆)
  • Subscripts in molecular formulas indicate the number of atoms of each element present in a single molecule

Additional Methods for Determining Formulas

  • Combustion analysis: A technique used to determine the empirical formula of organic compounds by completely burning the sample and measuring the masses of CO₂ and H₂O produced
  • Mass spectrometry: An analytical technique that can determine the molecular mass of a compound, which is crucial for confirming molecular formulas

Key Terms to Review (33)

Δoct: Δoct, also known as crystal field splitting energy, represents the energy difference between the higher-energy and lower-energy sets of d-orbitals in an octahedral coordination complex. It plays a crucial role in determining the electronic structure and properties of transition metal complexes.
Chemical equation: A chemical equation is a symbolic representation of a chemical reaction, showing the reactants and products with their respective quantities. It must obey the law of conservation of mass, meaning the number of atoms for each element must be equal on both sides.
Empirical formula mass: The empirical formula mass is the sum of the atomic masses of all atoms in an empirical formula. It represents the simplest ratio of elements in a compound.
Empirical formula: An empirical formula represents the simplest whole-number ratio of elements in a compound. It does not show the actual number of atoms, but rather the relative proportions.
Hydrogen bonding: Hydrogen bonding is a strong type of dipole-dipole interaction that occurs between molecules when hydrogen is covalently bonded to electronegative atoms like oxygen, nitrogen, or fluorine. This bond results in higher boiling and melting points for substances.
Law of constant composition: The law of constant composition states that a given compound always contains the same proportion of elements by mass. This principle is fundamental in understanding chemical formulas and reactions.
Molecular formula: A molecular formula indicates the types and numbers of atoms in a molecule of a substance. It shows the actual number of each type of atom present in a single molecule.
Molar mass: Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). It is calculated by summing the atomic masses of all atoms in the molecular formula.
Percent composition: Percent composition is the percentage by mass of each element in a compound. It is used to determine the relative amounts of elements in a chemical formula.
Orbital diagrams: Orbital diagrams are graphical representations of the electron configurations in atoms. They use boxes or lines to represent orbitals and arrows to represent electrons with their spins.
Stoichiometry: Stoichiometry is the calculation of reactants and products in chemical reactions using balanced chemical equations. It involves the use of molar ratios derived from these equations to predict quantities of substances consumed and produced.
Law of Constant Composition: The law of constant composition, also known as the law of definite proportions, states that the elements in a pure chemical compound are always present in the same proportions by mass, regardless of the source or method of preparation of the compound.
Hydrogen: Hydrogen is the simplest and lightest element in the periodic table, with a single proton and electron in its neutral state. It is a highly reactive nonmetal that plays a crucial role in various chemical processes and is a fundamental component of many compounds, making it a key topic across several areas of chemistry.
Molar Mass: Molar mass is the mass of one mole of a substance, expressed in grams per mole (g/mol). It is a fundamental concept in chemistry that relates the mass of a substance to the number of particles (atoms, molecules, or ions) it contains, and is essential for understanding chemical formulas, stoichiometry, and the behavior of gases.
Molecular Formula: The molecular formula is a concise representation of the elemental composition and relative abundance of atoms within a chemical compound. It provides the specific number and type of atoms that make up a molecule, conveying the true chemical identity of the substance.
Oxygen: Oxygen is a highly reactive nonmetallic element that is essential for most forms of life. It is the third most abundant element in the universe and the most abundant element on Earth's crust. Oxygen plays a crucial role in various chemical and biological processes, including respiration, combustion, and oxidation-reduction reactions.
Carbon: Carbon is a fundamental element that is essential for the formation of organic compounds and the sustenance of life. It is a versatile element that can form a wide range of structures and participate in numerous chemical reactions, making it a crucial component in various fields, including chemistry, biology, and materials science.
Empirical Formula: The empirical formula represents the simplest whole-number ratio of atoms of each element present in a compound. It provides the most basic representation of the composition of a compound without specifying the actual number of atoms present.
H: H is the chemical symbol for hydrogen, the lightest and most abundant element in the universe. It is a key component in many chemical processes and plays a crucial role in various topics in chemistry, including atomic structure, formula mass, and determining empirical and molecular formulas.
C: C, the third letter of the alphabet, is a versatile element that plays a crucial role in various chemical contexts, including formula mass calculations, empirical and molecular formula determinations, and the properties of phosphorus. As a fundamental building block of organic compounds, C is essential for understanding the fundamental principles of chemistry.
Glucose: Glucose is a simple sugar that serves as the primary source of energy for the body's cells. It is a monosaccharide, the most basic unit of carbohydrates, and plays a crucial role in the topics of formula mass, the mole concept, and determining empirical and molecular formulas.
Empirical Formula Mass: The empirical formula mass is the sum of the atomic masses of the elements present in the simplest whole number ratio that represents the composition of a compound. It provides information about the relative amounts of each element in a compound without specifying the actual number of atoms present.
Grams: Grams are a unit of measurement in the metric system that is used to quantify the mass or weight of an object or substance. It is a fundamental unit in chemistry and plays a crucial role in various chemical concepts and calculations.
Chemical Equation: A chemical equation is a concise representation of a chemical reaction, where the reactants (starting materials) are shown on the left side, an arrow indicates the direction of the reaction, and the products (resulting substances) are shown on the right side. Chemical equations provide a standardized way to communicate and understand the transformations that occur during chemical processes.
Mass Spectrometry: Mass spectrometry is an analytical technique that measures the mass-to-charge ratio of ions to determine the elemental composition and molecular structure of a sample. It is a powerful tool used in the context of determining empirical and molecular formulas.
Combustion Analysis: Combustion analysis is a quantitative analytical technique used to determine the elemental composition of organic compounds. It involves the complete oxidation of a sample through high-temperature combustion, allowing for the precise measurement of the amounts of carbon, hydrogen, and other elements present.
O: O, or oxygen, is a chemical element that is essential for life and plays a crucial role in various chemical processes, including the formula mass and mole concept, the determination of empirical and molecular formulas, and ionic bonding. As one of the most abundant elements on Earth, O is a highly reactive nonmetal that forms compounds with most other elements.
Moles: A mole is a unit of measurement in chemistry that represents a fixed number of particles, such as atoms, molecules, or ions. It serves as a bridge between the microscopic world of chemical reactions and the macroscopic world of observable quantities.
Whole-Number Ratio: A whole-number ratio is a relationship between two or more quantities expressed as a ratio of whole numbers. It represents the relative proportions of the quantities involved, where each term in the ratio is a whole number.
Atomic Masses: Atomic masses, also known as atomic weights, are the average masses of individual atoms of a particular element. They are fundamental properties that provide crucial information for determining the composition and structure of molecules and compounds.
Percent Composition: Percent composition refers to the relative amount of each element present in a chemical compound, expressed as a percentage of the total mass or mole fraction of the compound. It is a fundamental concept in chemistry that helps understand the makeup and properties of substances.
Elemental Composition: Elemental composition refers to the relative amounts and types of chemical elements present in a substance or material. It is a fundamental property that describes the atomic-level makeup of a compound or mixture.
Subscripts: Subscripts are small numbers or symbols written below and to the right of a chemical element or formula, which indicate the number of atoms of that element present in a compound. They are an essential component in representing the accurate chemical composition of a substance.