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

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Presentation transcript:

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

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.

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 = x gram Using this relative system, the mass of all other atoms can be assigned. Example 29 Si = 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 28 Si % 29 Si % 30 Si %

Atomic Mass x ( amu) = amu x ( amu) = 1.35 amu x ( amu) = amu 28 Si 29 Si 30 Si Average atomic mass for silicon = amu

The Mole The number of atoms in grams of 12 C can be calculated. One atom 12 C = amu = amu x (1.661 x g/amu) 1 atom = x g Number of atoms = g (1 atom / x g) = x atoms The number of atoms of any element needed to equal its atomic mass in grams will always be 6.02 x atoms, a quantity known as the mole. (6.02 x 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 12 C = grams mass of 1 mole of C = grams mass of 1 mole of Na = grams mass of 1 mole of H = grams mass of 1 mole of O = 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 = g/mol Molecular weight of O = g/mol Molecular weight of H = g/mol FW = ( ) g/mol = g/mol

Molarity Molarity is a the of concentration of an aqueous solution measured in units of moles per liter. 1 molar NaOH contains grams of NaOH in 1 liter of solution Written as 1M NaOH 0.1M NaOH contains g NaOH/liter of solution