Chemical Measurements:

Objectives:

    1.    Define the mole and explain its importance.

    2.    Explain the meaning of Avagadro's number.

    3.    Explain how molar mass relates the number of particles of a substance to the mass of a substance.

    4.    Distinguish between molar mass and formula mass.

Key Terms:

    Atomic mass    Formula mass    mole    Avagadro's number    Molar mass

Notes: (10-1)

Whether you are buying fruit at the store or checking your weight at home, the determination of weight is both important and structured.  This is especially the case when figuring out medications where the dose is determined based on the weight of the individual, the strength of the medication (molality) and the amount of the diluents (what it is mixed in).  To make these types of calculations it is important to know the difference between the different types of chemical masses.

Atomic Mass vs Formula Mass:

Atomic Mass

• The atomic mass number lists the mass in amu (atomic mass unit) notation.
• This number is based on the mass of the carbon-12 atom

Example:    H = 1amu    O = 16amu       C = 12 amu

Formula Mass

• Formula mass is determined by adding all of the masses of atoms in a molecule and reaction.
• Remember that the masses of the reactants need to equal the masses of the molecules and products.

Example:    H₂O = 18amu    CO₂ = 44amu    NaHCO₃ = 84amu

                    2H₂ +O₂ g 2H₂O (reactants and products each add up to 36amu)

Mole and Avagadro's number:

Since the amu has no real experimental significance in the laboratory setting, scientists needed to convert the atoms and molecules into a useful and predictable unit.  From this need the mole was born.  Later the mole was further defined by Avagdro as he equated  all moles to the exact number of atoms in 12 grams of carbon-12.  

(www.chem.vt.edu/RVGS/ACT/ Images/reader.gif)

• A molar mass of an atom is simply its am

(www.mpcfaculty.net/mark_bishop/ H2O_space_atomic_mass.jpg)

• The molar of a molecule is the combined atomic masses of its constituents (atoms) expressed in grams

(www.mpcfaculty.net/mark_bishop/ H2O_space_atomic_mass.jpg)

• For a single atom - A mole of an atom contains 6.02 x 10²³ atoms
• For a molecule - A mole contains 6.02 x 10²³ molecules and 6.02 x 10²³ times the number of atoms of that material

Example:     O = 16amu = 16g = 6.02 x 10²³ atoms

                      O₂ = 32amu = 32g = 6.02 x 10²³ molecules = 12.04 x 10²³atoms

Please see the following link for more information.

    http://gaia.fc.peachnet.edu/tutor/

Avagadro's number (6.02 x 10²³) established a direct relationship between the amu of a substance and the mole of a substance.