Transcript Mole of atoms

Section 6.1
Atoms and Moles
• How many atoms are there in a bucket of water? How
could we determine that without having to count them all
individually?
• We know that one atom of carbon reacts with one
molecule of oxygen to make one molecule of carbon
dioxide. If we have a sample of carbon how can we
predict how much oxygen to react with it? Can we do it by
counting out atoms?
C + O2 reacts to give CO2
• What do the atomic masses in the Periodic Table stand
for?
periodic table
Section 6.1
Atoms and Moles
A. Counting by Weighing
• We can count objects by knowing how much each one
weighs and weighing the sample
• Objects do not need to have identical masses to be
counted by weighing.
– All we need to know is the average mass of the
objects.
• To count the atoms in a sample of a given element by
weighing we must know the mass of the sample and the
average mass for that element.
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Section 6.1
Atoms and Moles
A. Counting by Weighing
Averaging the Mass of Similar Objects
Example: How can we count out 1000 jelly beans?
1. Not all jelly beans have the same mass.
2. Suppose we weigh 10 jelly beans and find:
3. Now we can find the average mass of a bean.
4. Finally we can multiply to find the mass of 1000
beans. We can then weigh out 1000 beans.
Section 6.1
Atoms and Moles
B. Atomic Masses: How much do Atoms Weigh?
• Atoms have very tiny masses so scientists
made a unit to avoid using very small
numbers.
• 1 atomic mass unit (amu) is 1/12 of the
mass of a 12C atom
• 1 amu = 1.66 10-24 g
How many amu’s does a 12C atom weigh?
How many grams does a 12C atom weigh?
Section 6.1
Atoms and Moles
B. Atomic Masses: Counting Atoms by Weighing
• The average atomic mass for an element is the
weighted average of the masses of all the isotopes of an
element.
• The Periodic Table shows the atomic mass of each
element in amu’s. This is different to the mass number
periodic table
What is the average atomic mass in amu’s of an atom of:
Barium
Fluorine
Xenon
Nickel
Section 6.1
Atoms and Moles
Calculate the Average Atomic Mass of Naturally
Occurring Iron:
Isotope
Atomic Mass
amu
Natural
Abundance %
Abundance x
Atomic Mass
54Fe
53.9396127
5.845
3.153
56Fe
55.9349393
91.754
51.323
57Fe
56.9353958
2.119
1.206
58Fe
57.9332773
0.282
0.163
Sum of the final column = 55.845 amu
Section 6.1
Atoms and Moles
C. The Mole
Multiple Units
• One dozen eggs is twelve eggs
• One class is twenty students
• One kilometer is 1000 meters
• One alphabet is …………?
• One Liter is ………..?
• One gross is………...?
Can you think of some multiple units?
Section 6.1
Atoms and Moles
End
Section 6.1
Atoms and Moles
C. The Mole
• One mole of anything contains 6.022 x 1023 units of that
substance.
– Avogadro’s number is 6.022 x 1023.
• A sample of an element with a mass equal to that
element’s average atomic mass (expressed in g)
contains one mole of atoms.
Section 6.1
Atoms and Moles
C. The Mole
Section 6.1
Atoms and Moles
Summary
• One amu is one-twelfth of the mass of a 12C atom
• One amu is close to the mass of one proton or
one neutron.
• One amu is a very small mass – 1.66 x 10-24 g
• One mole is 6.022 x 1023 units of anything
• One mole (of atoms) of an element will have a
mass in grams equal to the mass in amu of one
atom of that element
• “One Mole” refers to 6.022 x 1023 atoms or to the
atomic mass expressed in grams of an element
Section 6.1
Atoms and Moles
Some Questions………
• How many atoms are there in a bucket of water? How
could we determine that without having to count them all
individually?
• I know that one atom of carbon reacts with one molecule
of oxygen to make one molecule of carbon dioxide. How
many grams of oxygen react with 12 grams of carbon to
make carbon dioxide?
C + O2 reacts to give CO2
• How are the atomic masses in the Periodic Table
different to mass numbers?