Chapter 6: Chemical Composition – Study Notes for Introductory Chemistry

Chemical Composition

Introduction to Chemical Composition

Chemical composition refers to the types and amounts of elements that make up a compound. Understanding chemical composition is essential for quantifying substances, predicting reactions, and analyzing materials in chemistry.

The Mole Concept

Definition and Importance of the Mole

The mole is a fundamental unit in chemistry used to count atoms, molecules, or ions. One mole contains Avogadro’s number of particles, which is $6.022 \times 10^{23}$ units. This allows chemists to relate microscopic particles to macroscopic quantities.

• Avogadro’s Number: $6.022 \times 10^{23}$ particles per mole

• Application: Used to convert between number of particles and mass

• Example: Twenty-two copper pennies contain about 1 mole of copper atoms.

Counting by Weighing: Nails and Atoms

Counting large numbers of small objects, such as nails or atoms, is done by weighing. The process involves conversion factors, similar to counting nails by the pound.

• Conversion Factors: Used to relate mass to number of items

• Example: Converting pounds of nails to number of nails using weight per dozen and number per dozen.

Molar Mass and Atomic Mass

Atomic Mass Unit and Molar Mass

The atomic mass unit (amu) is defined as one-twelfth the mass of a carbon-12 atom. The molar mass of an element is the mass of 1 mole of its atoms, numerically equal to its atomic mass in grams.

• Atomic Mass: Mass of a single atom in amu

• Molar Mass: Mass of 1 mole of atoms in grams

• Example: Copper has an atomic mass of 63.55 amu; 1 mole of copper atoms has a mass of 63.55 grams.

Conversions: Moles, Grams, and Atoms

Converting Moles to Number of Atoms

To convert moles to number of atoms, multiply the number of moles by Avogadro’s number.

• Formula: $\text{Number of atoms} = \text{moles} \times 6.022 \times 10^{23}$

• Example: 3.5 mol He $\rightarrow$ number of He atoms

Converting Grams to Moles and Atoms

To convert grams to moles, divide the mass by the molar mass. To convert grams to atoms, first convert to moles, then multiply by Avogadro’s number.

• Formula: $\text{Moles} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}$

• Formula: $\text{Number of atoms} = \text{moles} \times 6.022 \times 10^{23}$

• Example: How many aluminum atoms are in a 16.2 g aluminum can?

Converting Between Grams and Moles of a Compound

The molar mass of a compound is the sum of the atomic masses of its constituent atoms. To convert between grams and moles of a compound, use the compound’s molar mass.

• Formula: $\text{Moles of compound} = \frac{\text{mass (g)}}{\text{molar mass (g/mol)}}$

• Example: Calculate the mass of 1.75 mol of water.

Chemical Formulas as Conversion Factors

Using Chemical Formulas to Relate Elements and Compounds

Chemical formulas indicate the ratio of elements in a compound, allowing conversion between moles of compound and moles of constituent elements.

• Example: 1 mol CCl4 contains 4 mol Cl.

• Application: Used to calculate the amount of an element in a compound.

Mass Percent Composition

Definition and Calculation

The mass percent composition of an element in a compound is the percentage of the total mass contributed by that element. It is calculated using the formula:

• Formula: $\text{Mass percent} = \frac{\text{mass of element in 1 mol}}{\text{mass of 1 mol of compound}} \times 100\%$

• Example: Mass percent of Cl in C2Cl2F2

Empirical and Molecular Formulas

Empirical Formula

An empirical formula gives the smallest whole-number ratio of atoms in a compound. The molecular formula is a whole-number multiple of the empirical formula.

• Example: Hydrogen peroxide: Empirical formula HO, molecular formula H2O2

• Calculation Steps:

  1. Write masses of each element.

  2. Convert masses to moles.

  3. Write a pseudoformula using mole values.

  4. Divide by the smallest subscript to get whole numbers.

Calculating Molecular Formula

The molecular formula is determined by multiplying the empirical formula by an integer n, found by dividing the molar mass by the empirical formula mass.

• Formula: $n = \frac{\text{molar mass}}{\text{empirical formula mass}}$

• Example: Fructose molecular formula calculation

Review and Learning Objectives

Summary of Key Concepts

• The mole allows counting atoms or molecules by weighing.

• Chemical formulas indicate the relative number of each kind of element in a compound.

• Empirical and molecular formulas can be determined from mass data and molar mass.

Learning Objectives

• Convert between moles and number of atoms.

• Convert between grams and moles.

• Convert between grams and number of atoms or molecules.

• Convert between grams and moles of a compound.

• Convert between mass of a compound and number of molecules.

• Convert between moles of a compound and moles of a constituent element.

• Convert between grams of a compound and grams of a constituent element.

• Use mass percent composition as a conversion factor.

• Determine mass percent composition from a chemical formula.

• Determine an empirical formula from experimental data.

• Calculate an empirical formula from reaction data.

• Calculate a molecular formula from an empirical formula and molar mass.