The greatest pleasure in life is doing what people say you
cannot do.
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
MOLAR MASS – THE MASS OF ONE MOLE OF
ITS CHEMICAL FORMULA UNIT.
THE MOLAR MASS CAN BE FOUND BY
ADDING THE MOLAR MASSES OF ALL
ELEMENTS PRESENT WITHIN THE
COMOUND.
EXAMPLE: MOLAR MASS OF H2SO4
2 H =
2 X 1 =
2
1 S =
1 X 32 = 32
4 O =
4 X 16 = 64
Molar mass
= 98 g/mole
CALCULATE THE MOLAR MASSES OF THE
FOLLOWING:
A. K3PO4
B. TiBr2
C. Fe2S3
D. CH3CH2OH
IN THE CASE OF IONIC COMPOUNDS,WE
SOMETIMES USE THE TERM FORMULA
UNIT MASS RATHER THAN MOLAR
MASS,AS TRUE MOLECULES DO NOT
EXIST IN IONIC COMPOUNDS.
ANOTHER LOOK AT FORMULAS AND MOLES.
MOLECULAR FORMULA – A COLLECTION OF
SYMBOLS REPRESENTING THE NUMBER OF
ATOMS OF EACH ELEMENT IN A MOLECULE
OF A COMPOUND.
EMPIRICAL FORMULA – THE SIMPLEST
WHOLE NUMBER RATIO OF THE ATOMS IN
A MOLEUCLE OF A COMPOUND.
IF WE ARE TALKING ABOUT AN IONIC
COMPOUND, THE FORMULA UNIT, OR
FORMULA, IS THE SAME AS THE EMPIRICAL
FORMULA.
FOR EXAMPLE, THE MOLECULAR FORMULA
FOR A SIMPLE SUGAR IS:
C6H12O6
THE EMPIRICAL FORMULA IS:
CH2O
IF YOU KNEW THE EMPIRICAL FORMULA,
AND YOU KNEW THE MOLAR MASS WAS
180, YOU COULD CALCULATE THE
MOLECULAR FORMULA.
THE EMPIRICAL FORMULA MASS WOULD BE:
1 C = 1 X 12 = 12
2 H = 2 X 1 = 2
1 O = 1 X 16 = 16
EMPIRICAL FORMULA MASS = 12 + 2 + 16 = 30
MOLAR MASS/EFM = 180/30 = 6
SO THE MOLAR MASS IS 6 TIMES THE
EMPIRICAL FORMULA MASS, SO THE
MOLECULAR FORMULA IS:
6 X CH2O = C6H12O6
MOLE/FORMULA PROBLEMS:
1) HOW MANY GRAMS OF MgCl2 ARE IN
1.5 MOLES?
2) HOW MANY MOLES OF Cl- ARE IN 0.255
g OF MgCl2 ?
3) HOW MANY ATOMS OF OXYGEN ARE
IN 75.0 g OF Fe2O3?
4) HOW MANY ATOMS OF C ARE IN 1.05 g
TOLUENE?
MASS PERCENT COMPOSITION – THE
RELATIVE MASS OF EACH ELEMENT
PRESENT IN A GIVEN COMPOUND.
PLEASE REMEMBER: EVERY CHEMICAL
COMPOUND HAS A DEFINITE CHEMICAL
FORMULA, WHICH INDICATES THAT EVERY
CHEMICAL COMPOUND HAS A DEFINITE
PERCENT MASS COMPOSITION.
EXAMPLE: CALCULATE THE % C IN
ACETALDEHYDE, C2H4O.
IF YOU KNOW THE % COMPOSITION, YOU
CAN CALCULATE THE EMPIRICAL FORMULA.
SOLID TARTARIC ACID, WHICH IS
PRODUCED IN THE FERMENTATION OF
WINE, HAS THE PERCENT COMPOSITION
32.01% C, 4.03% H, AND 63.96% O .
CALCULATE THE EMPIRICAL FORMULA.
Find an empirical formula for a substance with
53.28% C, 15.65% H, and 31.07% N.
Find the empirical formula for a compound
with 11.33% C, 45.29% O, and 43.38% Na.