OB: Percent composition by mass
and mole problem practice day
All your stuff, except pen and
reference tables on black tables
We’ll be working in groups of 2.
Put your stuff AWAY.
You need paper, pen,
reference tables
and calculators.
Question one:
What is the percent composition by mass of
aluminum in aluminum hydroxide?
Let’s round answers to 3 SF
Question one:
What is the percent composition by mass of
aluminum in aluminum hydroxide
Al(OH)3
Al 1 x 27 = 27
Al
27g
78g X 100% = 34.6%
O 3 x 16 = 48
O
48g
78g
X 100% = 61.5%
H
H
3g
78g
X 100% = 3.85%
3x1=3
78 g/mole
99.95% = 100.%
You find a box with a bar of metal that has stamped
into it PURE GOLD. The bar weighs 500. grams
EXACTLY. How many atoms of gold do you have?
You find a box with a bar of metal that has stamped
into it PURE GOLD. The bar weighs 500. grams
EXACTLY. How many atoms of gold do you have?
500. g Au
1
x
1 mole Au
197 g Au
= 2.54 moles Au
2.54 moles Au x 6.02 x 1023 atoms Au =
1 mole Au
1
2.54 x 6.02 x 1023 atoms Au = 15.3 x 1023 = 1.53
x 1024 atoms Au
If you have 50.0 grams of sodium hydroxide,
how many grams of those are oxygen?
If you have 50.0 grams of sodium hydroxide, how many
grams of those are oxygen?
% Comp by Mass
NaOH
Na 1x23= 23
O
1x16= 16
H
1x1 =
O
16g
40g
X 100% = 40%
1
40g/mole
50.0 g x .40 = 20.0 grams oxygen
Assume
WHAT IS THE
STP
346 L
neon
gas
Assume
STP
346 L
neon
gas
346 L Ne
1
15.4 moles Ne
1
x
x
1 mole Ne
22.4 L Ne
20 g Ne =
1 mole Ne
= 15.4 moles Ne
308 g Ne
Some bigger compounds are not in the simple whole number ratio because the
molecules are so big and often are made of repeating components. These larger
formulas can be “reduced” mathematically into simpler ratios, but this is just math,
these simplified formulas usually don’t exist.
For example
Glucose formula is C6H12O6
which has a 6:12:6 atomic ratio
John Dalton said “simple, whole number ratios” for compounds,
but this one exists commonly.
It’s reduced, 1:2:1 ratio can exist on paper, in our mind, but not
for real. These “reduced” to simple ratio formulas are called
Empirical formula: CH2O
EMPIRICAL FORMULAS
They’re math, not chemistry.
Let’s figure out some empirical
formulas now
•
•
•
•
•
•
•
•
•
C6H12O6 (glucose)
C8H18 (octane)
C24H48 (candle wax)
C2H2 (acetylene gas)
H2O2 (hydrogen peroxide)
C6H6 (cyclohexene)
C10H22 (decane)
C6H14 (hexane)
C5H10O5 (pentose)
Empirical formulas are
mathematical ratios of
atoms to atoms
in compounds, written
(CONFUSINGLY)
as chemical formulas
Let’s figure out some empirical
formulas now
•
•
•
•
•
•
•
•
•
C6H12O6
C8H18 (octane)
C24H48 (candle wax)
C2H2 (acetylene gas)
H2O2 (hydrogen peroxide)
C6H6 (cyclohexene)
C10H22 (decane)
C6H14 (hexane)
C5H10O5 (pentose)
•
•
•
•
•
•
•
•
•
CH2O
C4H9
CH2
CH
HO
CH
C5H11
C3H7
CH2O
If you find 11.200 moles of gold and gold is selling for
about $40.39/gram, are you rich or just happy, or does
it not matter all that much?
(hint, how many grams are in that many moles?
If you find 11.2 moles of gold and gold is selling for
about $40.39/gram, are you rich or just happy, or does
it not matter all that much?
11.200 moles Au x 197 g Au = 2206.4 g Au 3SF
1
1 mole Au
2206.4 g Au
x
1
_$40.39_
1 g Au =
$89,116
(wow)
Due SOON:
Mole Lab
Bubble Gum Lab
TUESDAY: No school