A Mole Is a Chemical Counting Unit Like a Dollar Is to Money

THE MOLE

A mole is a chemical counting unit like a dollar is to money.

A mole is the amount of pure substance that contains as many particles (atoms, ions, or molecules) as there are atoms in exactly 12 grams of carbon 12.

1 mole = 6.0221367 x 1023 particles

6.02 x 1023 is Avogadro’s number. Regardless of what substance you have, 1 mole always equals 6.02 x 1023 particles.

Molar mass is the mass of one mole of atoms of any element. Molar mass is expressed in units of grams per mole (g/mol).

The molar mass of an element is numerically equal to the atomic weight in AMUs (1 AMU = 1 g/mol).

The mole concept is the cornerstone of quantitative chemistry. The ratios of atoms expressed in a compound are mole ratios.

Conversions

(1)gram to mole conversions

X is given.

X1 is the molar mass (g/mol) from the periodic table.

Example:

Convert 2.51 g of Cu to moles of Cu.

(2) mole to gram conversion

Example:

Convert 0.0084 moles HNO3 to grams HNO3.

(2)number of particles (atoms, ions, molecules) in a sample

Example:

Convert 47 moles of He to atoms He.

Example:

Convert 1 mole of Cu(NO3)2 to molecules and grams.

Molecular Weight – is the sum of the atomic weights of atoms in a molecule.

Example: H2O = 18 AMUs

Formula Weight – is used for ionic compounds that do not exist as a molecule and is given in AMUs.

Example: NaCl = 58.8 AMUs

Compounds are combinations of atoms in small, whole number ratios. The smallest unit of a compound that retains all of the properties of the compound is a molecule.

The composition of a molecule can be represented by the molecular formula. A molecular formula expresses the number of atoms of each type of element in one molecule.

Example: K2O3 – 2 K atoms and 3 O atoms

Units of a molecule can be separated by parentheses.

Example: C6H4(OH)2

The modified molecular formula attempts

to show groupings of atoms in the molecule. It tells you more about the molecular arrangement.

Example: CH3CH2OH (This tells us more than C2H6O.)

A structural formula provides even more information about how the molecule is put together.

Example: H – C – C – OH

Some elements (nonmetals) exist as molecules in the elemental state.

Examples: Br2, I2, Cl2, N2, H2, S8, O2, P4, F2

Diatomic elements or molecules consist of two atoms of the same element covalently bonded.

You can recall the diatomic gases by remembering the name Brinclhof.

Percent composition of an atom in a molecule can be calculated if you know the formula for the compound and have a periodic table.

Mass % (weight %) = mass of one element in one mole of the compound / mass of one mole of the compound x 100%

Example:

Calculate the percent of each element in Pb(NO3)4 by mass.

Example:

Calculate a weight % for each element in a mixture that is 1.67 g Ce and 4.54 g I.

In general, you can easily describe composition by comparing one part of a mixture to the whole mixture.

Example:

In Pb(NO3)4, what is the percentage of each element in the given formula unit.

Atom % = # of atoms of one element / total # of atoms in compound x 100%