## Content in a Nutshell

The mole concept pervades all of chemistry. Since most quantitative chemical calculations are based on the mole, an understanding of the mole is essential to the study of chemistry. An understanding of how the mole relates to mass, number of entities (atoms, molecules, ions, etc.) and volume of a gas is included in this module. Formulas are most often interpreted in terms of moles rather than atoms, molecules or formula units (see Language of Chemistry section) since chemical reactions are generally carried out on a macroscopic scale.

The SI definition of mole is: "... the amount of substance of a system that contains as many elementary entities as there are atoms in 0.012 kg of carbon 12. When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particles, or specified groups of such particles."

Chemists use the mole (symbolized mol) to express amount of substance (elements, compounds, ions, electrons, etc.). Avogadro's number (6.02 x 1023 ) is the number of entities in one mole of a substance. Because it is easier to weigh large numbers of particles than to count them, chemists generally relate moles of a substance to mass rather than to the number of particles. The molar mass (mass of one mole) of a substance is the sum of the relative atomic masses (in grams) of the elements in a formula unit of the substance. Avogadro's number of formula units of a substance has a mass equal to the molar mass of the substance.

The subscripts in a formula can be interpreted as the number of moles of the element present in a mole of the compound. Calculations involving percent composition of a compound and empirical formula involve the mole concept. (The relationship of moles to stoichiometry is the subject of another SourceBook module.)

This topic lends itself to a historical presentation (see History section).

## Place in the Curriculum

1. One mole of a substance represents Avogadro's number (6.02 x 1023 ) of units of that substance.
2. The amount of a substance (expressed in moles) is related to the mass of substance, and for a gaseous substance, to its volume.
3. The mole is the basis of quantitative chemistry (percent composition, empirical and molecular formula determinations).

## Central Concepts

1. One mole of a substance represents Avogadro's number (6.02 x 1023 ) of units of that substance.
2. The amount of a substance (expressed in moles) is related to the mass of substance, and for a gaseous substance, to its volume.
3. The mole is the basis of quantitative chemistry (percent composition, empirical and molecular formula determinations).

## Related Concepts

1. Mass, volume
2. Formulas (empirical and molecular)
3. Atoms, molecules, formula units
4. Percent
5. Accuracy and precision
6. Gas behavior (particulate nature of matter, relationship of volume to number of molecules of gas)
7. Stoichiometry

## Related Skills

1. Measurement
2. Exponential numbers
3. Significant figure manipulation
4. Problem solving strategies (dimensional analysis, etc.)
5. Algebra
6. Use of balance
7. Volume measurements
8. Use of burner
9. Use of tongs to handle crucibles

## Performance Objectives

After completing their study of the mole concept, students should be able to:

1. define mole in terms of mass and number of entities.
2. explain the difference between the terms mole and molecule.
3. explain why chemists find it convenient to express the quantity of a substance in moles (rather than mass, volume, number of entities, or some other unit).
4. calculate the molar mass of a substance, given its formula.
5. calculate moles, mass or number of entities (atoms, molecules, formula units, etc.), given appropriate data.
6. calculate moles, mass or volume of a gas sample at STP, given appropriate data.
7. determine percent composition of a substance, given its formula.
8. determine empirical formula, given the percent composition of a substance or the relative masses of the elements in the substance.
9. determine the molecular formula of a substance, given its empirical formula and other appropriate data.