COUNTING UNITS FOR……..
atoms
These slides will help you navigate the
next section of the Counting Units Lab –
Experiment #5/Unit 2 (B):
Mole System.
Have a sheet of paper on which to
answer questions and work problems.
You will turn in your work at the end of
the class period.
Recall Part (A) Counting Unit – Model System:
1.We developed a relative mass scale for seeds
2.We determined, and named, a “counting unit” by
weighing out one relative mass (in grams) then
counting the number of seeds in that g-relative mass.
3.The counting unit was the same for each seed-type,
regardless of its size.
4.Moreover, the counting unit describes a relationship
between the relative mass and actual mass of a seed
(relative mass/actual mass).
5. The counting unit is a NUMBER …..
our counting unit was ________ (#), and was
named “_______________________”.
6. By knowing the relative mass of a seed-type,
and the # of seeds in a counting unit, we
found that we could easily measure out large
numbers of seeds,
e.g., if the relative mass of corn seeds was 6.0 g,
and the counting unit was 50 seeds, then we
could “measure” 500 seeds simply by
weighing out ________ g. (
)!
yes, the answer is 60.0 g
Chemists also use a counting unit:
to “measure” out numbers of atoms.
This counting unit is called the mole.
Just like all counting units, it is a NUMBER:
6.02 x 1023
large
This is quite a
number: write it
out on your paper. I’ll
wait………………………………………………………
Check it out….
Just a couple more:
If 10 billion chickens each laid 10
eggs per day, it would
take….................> 10 billion years to
produce 1 mole of eggs…
One mole of those chicken eggs
would fill all the oceans on
earth…. 30 million times….
The Periodic Table contains the relative masses of all the atoms……
Masses represent masses of atoms and are given in atomic mass units (amu).
Our mass of one grain of rice corresponds to the atomic mass unit of the periodic table.
The relative masses on the periodic table are based on the
“standard”: the carbon-12 isotope. The masses of atoms of all the
other elements are “relative” to the mass of an atom of carbon-12.
An atom of sulfur has a mass of ______ amu.
It’s mass is ______ X the mass of an atom of hydrogen.
Why
is the mole counting unit so very large?
(6.02 x 1023)
Because atoms are so very small, and it takes so very, very,
very many of them to give the relative mass, in grams.
We chemists measure atoms in grams..
When we weigh out a relative mass of atoms in grams,
we are measuring moles of atoms !!
A relative mass on the periodic table in amu = mass of one atom.
A relative mass on the periodic table in grams = mass of one mole
of atoms.
Measured mass
of one…..
Atom
Mole
Unit of
mass
Atomic mass unit
(amu)
Grams (g)
We call it …
Atomic mass
Molar mass
Atomic mass unit – used to describe masses of single atoms (protons + neutrons).
Grams – used to describe masses of moles of atoms.
Recall – from our Model System:
(a) the relative mass of any seed-type contained the
same number of seeds (the counting unit)
And so, in the Mole system:
(b) the relative mass of any atom-type contains the
same number of atoms (6.02 x 1023)
Where does the counting unit for atoms
(Avogadro’s number = 6.02 x 1023)
come from…………………………………………
Our model seed system showed us that the
counting unit describes a relationship between
the relative mass and the actual mass :
relative mass = counting unit
actual mass
Let’s start with Hydrogen:
How many atoms are in one relative mass of hydrogen (1.0 g)?
1. Relative mass = 1.0 gActual mass = 1.7 x 10-24 g (H has 1 p+)
2. relative mass = 1.0 g___________ = 5.9 x 1023 atoms
actual mass
1.7 x 10-24 g/atom
Mass of 1 proton (p+): 1.67 x 10-24 g
Mass of 1 neutron (n0): 1.67 x 10-24 g
Turn to page 2 of Exp’t 5/Unit 2(B) handout and complete table:
Use masses of protons and neutrons given to sum up actual mass for each
atom.
Atom
C
N
O
Cl
Ar
# protons
6
7
8
9
18
# neutrons
6
7
8
10
22
mass of atom
Complete the table for the elements given, then add 2 more
of your choice from the periodic table!
Check answers on the next slide…………………………………………
Element
Actual mass Relative mass
(g)
scale #1 (amu)
Relative mass # atoms in one
scale #2 (g)
relative mass
(rel mass/actual mass)
H
1.7 x 10-24
1.0 amu
1.0 g
5.9 x 1023
C
2.00 x 10-23
12.01 amu
12.01 g
6.01 x 1023
O
2.67 x 10-23
16.00 amu
16.00 g
5.99 x 1023
Cu
1.05 x 10-22
63.55 amu
63.55 g
6.05 x 1023
Pb
3.46 x 10-22
207.2 amu
207.2 g
5.99 x 1023
Avogadro’s number, the mole, 6.02 x 1023
converts the masses of atoms on the periodic table (amu)
into masses of moles of atoms (g) !
Some useful conversion factors:
1 mole = 6.02 x 1023 atoms
1 mol = relative mass (g)
e.g.,
C
Na
Solve:
12.01 g/mol atoms
1 mol atoms/23.0 g
How many grams in 0.50 mol S?
0.50 mol 32.1 g_______ = 16.1 g S
1 mol S atoms
Time for practice (use dimensional analysis to solve):
1. What is the relative mass (in grams) of phosphorous?
2. The mass of one atom of Mg is _____________; the
mass of one mole of Mg atoms is ____________.
3. What is the mass, in grams, of 3.0 mol of aluminum?
4. Determine the number of atoms in 25.5 g of silver.
5. How many atoms are in 3.45 mol of cobalt?
What is the mass of this number of atoms?
6. If you use 4.3 g of sodium in an experiment, how many
moles will you use?
7. Determine the number of moles in 125 g of zinc.
8. How many atoms are in 13.4 g of copper?
Compare this to the number of atoms in 13.4 g of
silver.
(check key on the board to confirm your answers)
When you have finished the practice problems,
turn in your paper and begin the Calculations
and Questions for part (B) of Exp’t 2.5.