Atomic Mass The Mole Molecular Weight Unit 8 Chemical
Quantities 1
Slide 3
Upon completion of this unit, you should be able to do the
following: Calculate the total mass of each element in a compound.
Calculate the molecular weight of compounds 2 Unit 8 Chemical
Quantities
Slide 4
Atomic Mass Most elements occur in nature as a mixture of
isotopes. ElementNumber of stable isotopes C 2 Cl 2 O 3 Fe 4 This
is one reason why atomic masses are not whole numbers. They are
based on averages. 3
Slide 5
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 35 Cl =
34.96885268 amu 4
Slide 6
Atomic Mass The average atomic weight of an element can be
calculated if the abundance of each isotope for that element is
known. Example - Chlorine is a mixture of two isotopes. Isotope
Mass (amu) Abundance 35 Cl 34.96885268 75.77 % 37 Cl 36.96590259
24.23 % 5
Slide 7
Atomic Mass 75.77 100 x (34.96885268 amu) = 26.50 amu 24.23 100
x (36.96590259 amu) = 8.95 amu 35 Cl 37 Cl Average atomic mass for
chlorine = 35.45 amu 6
Slide 8
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) = 6.02 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 Avogadros number) 7
Slide 9
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
Slide 10
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
Slide 11
Mass and Moles Example - Formula weight of NaOH MW of Na =
22.990 g/mol MW of O = 15.999 g/mol MW of H = 1.008 g/mol FW =
(22.990 + 15.999 + 1.008) g/mol = 39.997 g/mol 10
Slide 12
Pg 247-249 Suppose that after you graduate from college, you go
to work for a chemical company. You are working with a chemist to
improve the process that produces methanol from the reaction of
hydrogen and carbon monoxide. Your first assignment is to order
enough hydrogen and carbon monoxide to produce 6 kg of methanol (CH
3 OH). How would you determine how much of each material to order?
11 Chemical Equations
Slide 13
Write the unbalanced equation for the reaction. CO (g) + H 2
(g) CH 3 OH (l) Next, balance the equation. CO (g) + 2 H 2 (g) CH 3
OH (l) Understand that you can multiply this equation by any number
and it is still balanced. For example, multiply by 12 to get 12 CO
(g) + 24 H 2 (g) 12 CH 3 OH (l) This is still balanced. Since 12 is
a dozen we could also write this as 1 dozen CO (g) + 2 dozen H 2
(g) 1 dozen CH 3 OH (l) 12 Chemical Equations
Slide 14
We could also multiply the equation by a very large number, say
6.022 x 10 23. 6.022 x 10 23 [CO (g) + 2 H 2 (g) CH 3 OH (l)] or
6.022 x 10 23 CO (g) + 2 (6.022 x 10 23 ) H 2 (g) 6.022 x 10 23 CH
3 OH (l) Just as 12 is called a dozen, 6.022 x 10 23 is called a
mole so we could rewrite the equation as 1 mol CO (g) + 2 mol H 2
(g) 1 mol CH 3 OH (l) 13 Chemical Equations
Slide 15
Example 9.1, page 248 Propane (C 3 H 8 ) is a fuel commonly
used for cooking on gas grills. Propane reacts with oxygen to
produce heat and the products of carbon dioxide and water vapor.
This combustion reaction is represented by the unbalanced equation
C 3 H 8 (g) + O 2 (g) CO 2 (g) + H 2 O(g) Give the balanced
equation and state the meaning of the equation in terms of moles.
1C 3 H 8 (g) + 5O 2 (g) 3CO 2 (g) + 4H 2 O (g) 1 mol of C 3 H 8
reacts with 5 mol of O 2 to form 3 mol of CO 2 and 4 mol of H 2 O
14 Chemical Equations
Slide 16
Now use the balanced equation to predict the moles of products
that a given number of reactants will yield. Consider the
decomposition of water into hydrogen and oxygen. 2 H 2 O (g) 2 H 2
(g) + O 2 (g) The equation tells you that 2 mol of H 2 O will
decompose to yield 2 mol of H 2 and 1 mol of O 2. 15 Chemical
Equations