atomic masses how do you measure something too small to be seen? a scale is needed – what would be...
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Atomic Masses
How do you measure something too small to be seen?
A scale is needed – what would be a good standard?
12
C6
1 atomic mass unit (AMU) = 1/12 mass of Carbon-12
What would be the atomic mass of an element that was twice as heavy as Carbon-12?
24
6
What about one that is half as heavy?
The average atomic mass is NOT the mass number…
The Mass Spectrometer
- Gas state atoms are ionized at low pressure
- The ions are accelerated toward a magnet
- Magnet deflects them from straight line
- Deflection is inverse of atomic mass
- Atoms can be compared by weight
For example:
What is the mass of Fluorine?
1.583 X 12.00amu = 19.00amu
It takes a magnet 1.583 times as strong to deflect a stream of Fluorine atoms as it does a stream of Carbon-12 atoms
Fluorine is unique
All Fluorine atoms have 9 protons and 10 neutrons
All other atoms have several isotopes
In order to determine the atomic mass –
The relative amounts of each isotope must be accounted for
When a pure sample of Chlorine is passed through the device – the following graph is obtained
What does this tell us?
Cl-35 34.97amu 75.53%
Cl-37 36.97amu 24.47%
Atomic mass Abundance
Atomic mass
- includes the fact that not all samples of elements are like Fluorine
- not all abundances are like Chlorine
- the periodic table value is the average mass of all of those elements in the universe
- the units are amu’s
Q: How is the periodic table value calculated?
What the?...
Y1 and Y2 are the masses of each isotope
%Y1 and %Y2 are the abundances of each isotope
Bromine is a red-orange liquid that has an average atomic mass of 79.90amu.
It’s name is derived from the Greek “bromos” which means “stench”.
It has two naturally occurring isotopes; Br-79 (78.92 amu) and Br-81 (80.92 amu).
What is the abundance of the heavier isotope?
Strategy: the two isotopes have to add up to 100%. If Br-81 = X, then Br-79 must equal?...
100 - X
The heavier isotope has an abundance of 49%
Q: If 75% of the Chlorine in the universe is Cl-35 and the rest is Cl-37;
should the table entry for Chlorine be closer to 35 or 37?
Q: What is Chlorine’s atomic mass?
35
35.453 amu
Masses of atoms and Avagadro’s number
Consider H and He
Helium is about 4 times as heavy as Hydrogen
A sample of 100 Helium atoms should weigh about 4 times as much as 100 Hydrogen atoms
A sample of a million Helium atoms should weight about 4 time as much as a million Hydrogen atoms
By that thinking: 4 grams of Helium should contain the same number of atoms as 1 gram of Hydrogen
Specifically: 4.003 grams of Helium contains the same number of atoms as 1.008 grams of Hydrogen
** A sample of any element with a mass in grams equal to its atomic mass contains the same
number of atoms (NA), regardless of the identity of the element **
The numerical value of NA is 6.022x1023 and is known a
“Avagadro’s Number”
It was obtained by experiments comparing the weights of various
gases whose relative weights were already known
Q: How many atoms are in a ten gram sample of Titanium?
10.0 g Ti x48 g Ti
1 mole Tix
1 mole Ti
6.022 x 1023 atoms
=
1.26 x 1023 atoms
Q: How many atoms are in a ten gram sample of Titanium?
Avagadro’s Number worth of anything is called a “mole”
The mass of one mole of a compound is numerically equal to the sum of all of the atoms in the formula
Sugar is C12H22O11
Baking Soda is NaHCO3
Copper nails are just Cu
Mole – Mass Conversions
m = MM x nOne of the most important
equations used in chemistry!!!
(m) is the mass in grams
(MM) is the molar mass
(n) is the number of moles
m = MM x n
Aspirin is a compound called acetylsalicylic acid (ASA).
Its formula is C9H8O4
What is the mass of 0.509 moles of ASA?
MM = 180 g/mol
C = 12 x 9 = 108
H = 1 x 8 = 8
O = 16 x 4 = 64
m = MM x n
= (180 g/mole) x (0.509 moles)
= 91.6 g
Same problem done MY way
O.509 moles C9H8O4 x1 mole C9H8O4
180 g C9H8O4= 91.6 g
Some problems are harder than others…
A 1.00 g sample of aspirin contains 75.2% by mass of C9H8O4
How many moles of ASA are in the sample?
Mass of ASA? 0.752 x 1.00g = 0.752 g
0.752 g C9H8O4 x180 g C9H8O4
1 mole C9H8O4= 0.00418 moles
How many Carbon atoms are present in a 12.0 g sample of
C9H8O4?
12.0 g C9H8O4x
180 g C9H8O4
1 mole C9H8O4x
1 mole C9H8O4
6.02 x 1023 moleculesx
1 molecule
9 C atoms
3.61 x 1023 Carbon atoms
There is only ONE problem in chemistry!!!