Download Amu, Atomic Mass, Mole, they`re relative

Amu, Atomic Mass, Mole,
they’re relative
but neatly related!!!
In the early days
how much does an element weigh?
how much does an atom weigh?
Just take a stab…
“I decree that hydrogen will have a standard mass of 2!”
Stanislao Cannizzaro (1826–1910),
Using Hydrogen as the relative standard
• too small –
• too much error
Think bigger – Oxygen sixteen!
• it forms chemical compounds with many other
elements,
• lowest whole number and still have hydrogen > 1
Discovery of oxygen 17 and 18 – oops!!
•no problem for the chemists
(1/16th of the average mass of oxygen)
•Physicists, however, dealing with atoms required
a unit that distinguished between isotopes.
(1/16th of the mass of an atom of a particular isotope of oxygen)
In 1956, Alfred Nier (at the bar in the Hotel
Krasnapolski in Amsterdam) suggested that the
atomic weight scale be based on carbon-12.
•Physicists could care less
•Chemists would have to adjust all their atomic
masses by 42 ppm!
•Six years later chemists and physicists agreed
on C12
Amadeo Avogadro
• Italian physics professor
• his law – “equal volumes of
different gases at the same
temperature and pressure contain
equal numbers of atoms or
molecules.”
Equal Volumes of gases
O2
N2
Cl2
T = 0oC
P = 1 atmosphere
Equal Number of Particles
O2
N2
Cl2
but mass is not the same!
O2
2 x 16g
N2
2 x 14g
Cl2
2 x 35g
Definition!
A sample of any element with a mass equal to that
element's atomic mass in grams contains
one mole of atoms (6.02 x 1023 atoms)
it’s also called the gram atomic mass or gam
Practice:
1. One mole of chromium weighs ______ grams.
2. One mole of chromium contains _______ atoms.
3. The gam of chromium is _________.
1. 0.5M of chromium weighs ________ grams.
2. 0.5M of chromium contains __________ atoms
Definition!
A sample of any compound with a mass equal to that
compound’s molecular mass (in grams) contains
one mole of molecules (6.02 x 1023 molecules)
It’s also called the gram molecular mass or gmm.
Practice:
1. One mole of water weighs ______ grams.
2. One mole of water contains _______ molecules.
3. One mole of water contains ______ atoms of H
4. One mole of water contains ______ atoms of O
5. The gmm of water is ________
1.
2.
3.
4.
0.2M of water weighs ________ grams.
0.2M of water contains __________ molecules
0.2M of water contains ______ grams of H
0.2M of water contains ______ molecules of H
Definition!
A sample of any ionic compound with a mass equal to that
compound’s unit formula mass (in grams) contains
one mole of formula units (6.02 x 1023 molecules)
It’s also called the gram formula mass or gfm.
Practice:
1. How many moles are there in 320 grams of CoCl2?
2. How many grams are there in 4.2M Ca(NO3) 2 ?
1. How many molecules are there in 4.2M Ca(NO3) 2 ?
2. 2.45 x 1023 molecules of CH4 are ______ moles?
Practice:
1. One mole of table salt weighs ______ grams.
2. One mole of table salt contains _______ formula units.
3. One mole of table salt contains ______ atoms of Na
4. One mole of table salt contains ______ atoms of Cl
1. 0.75M of NaCl weighs ________ grams.
2. 0.75M of NaCl contains __________ formula units
In Summary
Mass is a way to count numbers of particles
•
A mole represents 6.02 x 1023 particles, be they
atoms, molecules, formula units or whateva
•
The mass of 1 mole of a substance is =
6.02 x 1023 particles x mass of one particle
•
•
•
for elements gam = gram atomic mass
for molecular compounds  gmm = gram molecular mass
for ionic compounds  gfm = gram formula mass
Volume of 1 Mole of Gas
STP = 0oC and 1 atmosphere
One Mole of any gas occupies 22.4 liters at STP
Practice – Part I:
1. One mole of gaseous oxygen weighs ______ grams.
2. One mole of gaseous oxygen contains _______ molecules
3. One mole of gaseous oxygen occupies_______ liters
Practice – Part II:
1. 0.75M of gaseous oxygen weighs ________ grams.
2. 0.75M of gaseous oxygen contains __________ molecules.
3. 0.75M of gaseous oxygen occupies _______ liters at STP.