Chemical Quantities and Moles

Section 12.1

Counting Particles of Matter

• Define the mole as a counting number.

• Relate counting particles to weighing samples of substances.

• Solve stoichiometric problems using molar mass.

Section 12.1

Counting Particles of Matter

Avogadro’s principle: equal volumes of gases at the same temperature and pressure contain equal numbers of particles

Section 12.1

Counting Particles of Matter

stoichiometry

mole

Avogadro’s number

molar mass

molecular mass

formula mass

A mole always contains the same number of particles; however, moles of different substances have different masses.

Section 12.1

• Stoichiometry is the study of quantitative relationships between reactants and products in a chemical reaction.

Section 12.1

Stoichiometry (cont.)

• The group or unit of measure used to count number of atoms, molecules, ions, or formula units is the mole (abbreviated mol).

• The number of things in a mole is 6.02 × 1023 and is called Avogadro’s number.

Section 12.1

Molar Mass• Using moles, macroscopic

measurements, such as mass, can be related to the molecules involved in a chemical reaction.

• The mass of one mole of a pure substance is called its molar mass.

Section 12.1

Molar Mass (cont.)

• The molar mass of an element is simply the average atomic mass of that element stated in grams rather than atomic mass units.

Section 12.1

Molar Mass (cont.)

• The molecular mass of a covalent compound is the mass in atomic mass units of one molecule.

– Its molar mass is the mass in grams of one mole of its molecules.

Section 12.1

Molar Mass (cont.)

• The formula mass of an ionic compound is the mass in atomic mass units of one formula unit.

– Its molar mass is the mass in grams of one mole of its formula units.

Section 12.1

Molar Mass and Stoichiometry

• The concept of molar mass makes it easy to determine the number of particles in a sample of a substance by simply measuring the mass of the sample.

• Use the molar mass to convert mass to moles or moles to mass.

Section 12.2

Using Moles

• Predict quantities of reactants and products in chemical reactions.

• Determine mole ratios from formulas for compounds.

• Identify formulas of compounds by using mass ratios.

Section 12.2

Using Moles

mole: group or unit of measure used to count numbers of atoms, molecules, or formula units of a substance

Section 12.2

Using Moles

molar volume

ideal gas law

theoretical yield

percent yield

empirical formula

Balanced chemical equations relate moles of reactants to moles of products.

Section 12.2

Using Molar Masses in Stoichiometric Problems

• Balanced chemical equations and moles can be used to predict the masses of reactants or products.

Section 12.2

Using Molar Masses in Stoichiometric Problems (cont.)

Section 12.2

Using Molar Volumes in Stoichiometric Problems

• The molar volume of a gas is the volume that a mole of gas occupies at a pressure of one atmosphere (equal to 101 kPa) and a temperature of 0.00°C.

Section 12.2

• The ideal gas law describes the behavior of an ideal gas in terms of pressure (P), volume (V), temperature (T), and number of moles of gas, n.

PV = nRT (R represents the ideal gas constant)

Ideal Gas Law

Section 12.2

• The amount of product of a chemical reaction predicted by stoichiometry is called the theoretical yield.

Theoretical Yield and Actual Yield

• The percent yield of a reaction is the ratio of the actual yield to the theoretical yield.

• Percent composition of a compound can be determined from its chemical formula.

Determining Mass Percents

Composition of Geraniol

Section 12.2

• To determine a chemical formula, find the relative numbers of each of the atoms in the formula unit of the compound.

Determining Chemical Formulas

• The formula of a compound having the smallest whole-number ratio of atoms in the compound is called the empirical formula.

Section 12.2

Determining Chemical Formulas (cont.)

Section 12.2

• The empirical formula of a compound can be determined from its percent composition.

• You can calculate the empirical formula from percent by mass by assuming you have 100.00 g of the compound. Then, convert to mass of each element to moles.

Determining Chemical Formulas

Section 12.2

• Chemical formulas for most ionic compounds are the same as their empirical formulas.

• However, the empirical formula is not always the chemical formula.

Determining Chemical Formulas

Section 12.2

• Many different covalent compounds have the same empirical formulas because atoms can share electrons in different ways.

• The chemical formula of a compound can be determined if the molar mass and the empirical formula are known.

Determining Chemical Formulas

Key Concepts

• Stoichiometry relates the amounts of products and reactants in a chemical equation to one another.

• The mole is a unit used to count particles of matter. One mole of a pure substance contains Avogadro’s number of particles, 6.02 x 1023.

• Molar mass can be used to convert mass to moles or moles to mass.

Key Concepts• A balanced chemical equation provides mole ratios of the

substances in the reaction.

• Percent composition can be determined from the chemical formula of a compound; the empirical formula of a compound can be determined from its percent composition.

• The chemical formula of a compound can be determined if the molar mass and the empirical formula are known.

• The ideal gas law is expressed in the following equation PV=nRT.

• Percent yield measures the efficiency of a chemical reaction.