Atomic MassThe Mole

Atomic WeightFormula Weight

Molarity of a Solution

Atomic Mass• Most elements occur in nature as a mixture of

isotopes.

Element Number of stable isotopesC 2O 3Fe 4

• This is one reason why atomic masses are not whole numbers. They are based on averages.

Atomic Mass• As a reference, we use the atomic mass unit

(amu), which is equal to 1/12th of the mass of a 12C atom.

• One atomic mass unit = 1.661 x 10-24 gram

• Using this relative system, the mass of all other atoms can be assigned.

Example 29Si = 28.976 4947 amu

Atomic Mass• The average atomic weight of an element can be

calculated if the abundance of each isotope for that element is known.

Example - Silicon is a mixture of three isotopes. Isotope Mass (amu) Abundance

28Si 27.9769265 92.23 %29Si 28.9764947 4.67 %30Si 29.9737702 3.10 %

Atomic Mass• 92.23• 100

• x (27.9769265 amu) = 25.80 amu

• 4.67• 100

• x (28.9764947 amu) = 1.35 amu

• 3.10• 100

• x (29.9737702 amu) = 0.929 amu

• 28Si

• 29Si

• 30Si

• Average atomic mass for silicon = 28.08 amu

The Mole• The number of atoms in 12.000 grams of 12C can be

calculated. • One atom 12C = 12.000 amu

= 12.000 amu x (1.661 x 10-24 g/amu)1 atom = 1.993 x 10-23 g

• Number of atoms = 12.000 g (1 atom / 1.993 x 10-23 g) = 6.021 x 1023 atoms

• The number of atoms of any element needed to equal its atomic mass in grams will always be

6.02 x 1023 atoms, a quantity known as the mole.(6.02 x 1023 is also known as Avogadro’s number)

The Mole• One mole equals the atomic mass in grams of

an element mass of 1 mole of 12C = 12.000 grams

mass of 1 mole of C = 12.011 grams

mass of 1 mole of Na = 22.990 grams mass of 1 mole of H = 1.008 grams

mass of 1 mole of O = 15.999 grams

Mass and Moles

Atomic mass or molecular weight (MW) The average mass of an atom in an element,

expressed in atomic mass units (amu) or grams / mole.

Formula mass (or formula weight FW) The total mass for all atoms in a compound.

Mass and Moles

Example - Formula weight of NaOH• Molecular weight of Na = 22.990 g/mol• Molecular weight of O = 15.999 g/mol • Molecular weight of H = 1.008 g/mol• FW = (22.990 + 15.999 + 1.008) g/mol = 39.997 g/mol

Molarity

• Molarity is a the of concentration of an aqueous solution measured in units of moles per liter.

• 1 molar NaOH contains 39.997 grams of NaOH in 1 liter of solution

• Written as 1M NaOH• 0.1M NaOH contains 3.9997 g NaOH/liter of

solution