1. Atomic Mass The Mole Atomic Weight Formula Weight Molarity of a Solution

2. Atomic Mass Most elements occur in nature as a mixture of isotopes. ElementNumber of stable isotopes C 2 O 3 Fe 4 This is one reason why atomic masses are not whole numbers. They are based on averages.

3. Atomic Mass As a reference, we use the atomic mass unit (amu), which is equal to 1/12 th of the mass of a 12 C 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 29 Si = 28.976 4947 amu

4. 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 28 Si27.976926592.23 % 29 Si28.9764947 4.67 % 30 Si29.9737702 3.10 %

5. 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 28 Si 29 Si 30 Si Average atomic mass for silicon = 28.08 amu

6. The Mole The number of atoms in 12.000 grams of 12 C can be calculated. One atom 12 C = 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 10 23 atoms The number of atoms of any element needed to equal its atomic mass in grams will always be 6.02 x 10 23 atoms, a quantity known as the mole. (6.02 x 10 23 is also known as Avogadro’s number)

7. The Mole One mole equals the atomic mass in grams of an element mass of 1 mole of 12 C = 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

8. 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.

9. 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

10. 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