6: Chemical Composition

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6.1: The Mole
Because of their small size, it is impractical to talk about individual atoms and molecules when considering amounts of matter. Instead, the quantity of a mole was developed as a useful way to count objects in chemistry.
6.2: Molecular Mass and Formula Mass
Formula masses of ionic compounds and molecular masses of molecular compounds can be determined from the masses of the atoms in their formulas.
6.3: Counting Objects By Weighing
The size of molecule is so small that it is physically difficult if not impossible to directly count out molecules. However, we can count them indirectly by using a common trick of "counting by weighing."
6.4: Molar Mass
In chemistry, it is impossible to deal with a single atom or molecule because we can't see them or count them or weigh them. Instead, we quantify them using moles. When the mole was determined, it was chosen so that the numerical value of the mass of one mole of an atom is the same as the atomic mass expressed in units of grams. When the average mass on the periodic table is expressed in units of grams, it is referred to as the molar mass.
6.5: Mole Calculations
The molar mass of a substance is the sum of the average molar masses of the atoms that compose the substance. The molar mass of a substance can be used as a conversion factor between moles of the substance and its mass.
6.6: Counting Particles By Weighing
Because the mole is a defined quantity, conversion factors can be created to convert between moles of a substance and the individual pieces--atoms, molecules, etc.--of the substance. When combined with the molar mass of the substance, it is possible to relate a mass of the substance in the lab to the number of individual pieces of that substance and vice versa.
6.7: Using Chemical Formulas as Conversion Factors
Using formulas to indicate how many atoms of each element we have in a substance, we can relate the number of moles of molecules to the number of moles of atoms. In any given formula the ratio of the number of moles of molecules (or formula units) to the number of moles of atoms can be used as a conversion factor.
6.8: Percent-By-Mass Composition
Chemists often need to know what elements are present in a compound and in what percentage. The percent composition is the percent by mass of each element in a compound.
6.9: Mass Percent Composition from a Chemical Formula
The percent composition of a compound can also be determined from the formula of the compound. The subscripts in the formula are first used to calculate the mass of each element in one mole of the compound. That is divided by the molar mass of the compound and multiplied by 100%.
6.10: For Future Use
6.11: Exercises

Thumbnail Chapter 6: A mole and Avogadro's number (adaptation by ChemLancer from NobleDame via tenor).

This page is shared under a CK-12 license and was authored, remixed, and/or curated by Melissa Alviar-Agnew, Henry Agnew, Vicki MacMurdo (Anoka-Ramsey Community College), and Lance S. Lund (Anoka-Ramsey Community College).

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