Everything is within walking distance if you have
the time.
- Steven Wright -
DALTON’S ATOMIC THEORY
ALL MATTER IS COMPOSED OF ATOMS.
ATOMS CANNOT BE MADE OR DESTROYED (IN
AN ORDINARY CHEMICAL REACTION).
ALL ATOMS OF THE SAME ELEMENT ARE
IDENTICAL.
DIFFERENT ELEMENTS WOULD HAVE
DIFFERENT TYPES OF ATOMS.
CHEMICAL REACTIONS OCCUR WHEN ATOMS
ARE REARRANGED.
COMPOUNDS ARE FORMED FROM A
COMBINATION OF TWO OR MORE ELEMENTS.
WE CAN REPRESENT CHEMICAL COMPOUNDS
USING SYMBOLS FOR THE ELEMENTS WITH
SUBSCRIPTS AFTER THE SYMBOL
REPRESENTING THE NUMBER OF ATOMS IN
THE COMPOUND. FOR EXAMPLE:
H2SO4
# HYDROGEN ATOMS
SULFURIC ACID
# OXYGEN ATOMS
ONE SULFUR ATOM
WE CAN ALSO USE OUR SYMBOLS TO
REPRESENT CHEMICAL REACTIONS. FOR
EXAMPLE:
3H2O + 2Fe  Fe2O3 + 3H2
THE ABOVE EQUATION TELLS YOU THAT 3
WATER MOLECULES WILL REACT WITH 2
IRON ATOMS TO YIELD ONE RUST
MOLECULE AND 3 HYDROGEN MOLECULES.
THE SUBSTANCES ON THE LEFT ARE
REACTANTS, AND THE SUBSTANCES ON
THE RIGHT ARE PRODUCTS.
3H2O + 2Fe  Fe2O3 + 3H2
THE EQUATION IS BALANCED. IN OTHER
WORDS, THERE ARE THE SAME NUMBER OF
ATOMS OF EACH TYPE ON THE LEFT AS ON
THE RIGHT.
MATTER IS NEITHER CREATED OR DESTROYED
IN A CHEMICAL REACTION. IT IS JUST
REARRANGED. THIS IS THE LAW OF
CONSERVATION OF MASS.
NOW, WE WOULD LIKE TO BE ABLE TO
CALCULATE HOW MANY ATOMS OF EACH
TYPE ARE INVOLVED IN DIFFERENT
SUBSTANCES. WE CAN FIGURE OUT HOW
MANY GRAMS OF EACH SUBSTANCE ARE
INVOLVED IN DIFFERENT COMPOUNDS.
BUT, WE CAN’T WEIGH INDIVIDUAL
ATOMS.
1 ATOMIC MASS UNIT = 1.67 X 10-24 grams
ROUTINE MASS DETERMINATIONS ARE
VERY DIFFICULT MUCH BELOW 10-6 grams.
WHAT WE CAN DO IS TO WORK WITH
LARGER QUANTITIES OF MATTER THAT
CONTAIN EQUAL NUMBERS OF ATOMS.
IF YOU LOOK AT THE ATOMIC MASSES
GIVEN ON THE PERIODIC TABLE, IT
TURNS OUT THAT IF YOU TAKE THESE
MASSES IN GRAMS, THEY WILL CONTAIN
THE SAME NUMBER OF ATOMS.
THIS NUMBER IS REFERRED TO AS
AVOGADRO’S NUMBER OR ONE MOLE.
6.023 X 1023 = 1 mole
FOR EXAMPLE, IF WE KNOW THAT 2
GRAMS OF HYDROGEN REACT WITH 16
GRAMS OF OXYGEN TO FORM WATER,
1 gram OF HYDROGEN = 1 mole HYDROGEN
SO, 2 g H = 2 moles
16 grams OXYGEN = 1 mole OXYGEN
IF 2 moles H REACTS WITH 1 mole OXYGEN,
2 atoms of H REACT WITH 1 atom OXYGEN,
SO THE FORMULA WOULD BE H2O
ANOTHER WAY YOU COULD LOOK AT THAT
IS:
1 gram H
16 gram O
2 X (6.023 X 1023) H + (6.023 x 1023) O to
give water
Or dividing by (6.023 x 1023), 2 H + 1 O
gives water
THE ATOMIC WEIGHT (MASS) IN GRAMS
CONTAINS 1 MOLE OF ATOMS OF THAT
ELEMENT.
THIS CONCEPT IS GOING TO ALLOW US
TO WORK WITH AND DETERMINE
CHEMICAL FORMULAS AND TO
UNDERSTAND CHEMICAL REACTIONS.
WE CAN DEAL WITH KNOWN NUMBERS
OF ATOMS WITHOUT HAVING TO
DETERMINE OR WEIGH INDIVIDUAL
ATOMS.
IN THE SI SYSTEM, A MOLE IS DEFINED
AS THE NUMBER OF ATOMS IN EXACTLY
12 GRAMS OF CARBON-12.
MOLE CALCULATIONS
FIRST, LET’S PRACTICE CALCULATING MOLAR
MASS (the mass of one mole of a substance in
grams).
CALCULATE THE MOLAR MASS OF CARBON
DIOXIDE, CO2
The atomic mass of C = 12.01
The atomic mass of O = 16.00
The molar mass of CO2 = 12.01 g/mol +
2(16.00 g/mol) = 12.01+ 32 = 44.01 g/mol
http://www.slideshare.net/robertgist/mole-calculations-aeg3 - GOOD REF.
CALCULATE THE MOLAR MASS OF Fe2O3
Atomic mass of Fe = 55.85 g/mol
Atomic mass of O = 16.00 g/mol
Molar mass of Fe2O3 = 2(55.85) + 3(16.00)
= 111.70 + 48
= 159.70 g/mol
Another good online tutorial - http://www.onlinemathlearning.com/molecalculation.html
HOW MANY GRAMS OF H2SO4 ARE IN 3
MOLES?
Atomic mass H = 1.01 g/mol
Atomic mass S = 32.07 g/mol
Atomic mass O = 16.00 g/mol
Molar mass H2SO4 = 2(1.01) + 32.07 + 4(16.00)
= 2.02 + 32.07 + 64.00
= 98.09 g/mole
Mass in 3 moles = 3 moles x 98.09 g/mole
= 294.27 g
HOW MANY GRAMS OF ACETIC ACID,
C2H4O2, ARE IN 0.3 MOLE?
Atomic mass C = 12.01 g/mole
Atomic mass H = 1.01 g/mole
Atomic mass O = 16.00 g/mole
Molar mass = 2(12.01) + 4(1.01) + 2(16.00)
= 24.02 + 4.04 + 32.00
= 60.06 g/mole
Grams in 0.3 mole = 0.3 mole x 60.06 g/mole
= 18.02 g
NOTE: Sometimes molar mass is called formula
mass.