Download - Chapter 6: Moles, Molar Mass, Percent Composition and Formulas From moles to mass and to the moon!
Chapter 6: Moles, Molar Mass, Percent Composition and
Formulas
From moles to mass and to the moon!
AMU (Atomic Mass Units)
The mass of Carbon-12 is 12 AMU.But wait, when I look on the periodic table,
the atomic mass is listed as 12.01078 AMU??? WHY? Why, cruel world?
WwWWwwhhHHHhhyyYYYYyy???
6.1 Atoms and Moles
Avogadro’s Number 6.022 x 1023
Avogadro discovered that there are 6.022 x 1023 atoms in 1 gram of hydrogen.
Amedeo Avogadro
Count Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e Cerreto
Be able to explain and use the concept of the “mole”
This number is called a “mole.”
The word “mole”is just like the word “dozen”. Dozenmeans “12”. You can have a dozen of anything. You can also have a mole of anything.
Hmmm… I shall call 6.022 x 1023... a “mole”. Yes…that has a nice ring to it.
So How Big is a “MOLE”
Ummm… NO!Here it is written out602,200,000,000,000,000,000,000That’s 602 billion groups of a trillion!Let’s just do an example with paper
clips.If you have a mole of paper clips and
made them into a chain, how many times could you go to the moon and back with your chain?
Don’t be cruel now…
Aaaiiiee
Assume a paper clip ( still folded) is about 3 cm long.
To find the total distance of the paper clips we use the following equation:
mole
cmx
mole
clipsxx
clip
cm 2423
10 806.1 1
1002.63
Notice the unit “clips” cancels!!! Isn’t that Great…
Anyone…
Anyone see the greatness???
Man I love Conversions!
The moon is 382,171 km from Earth, so to the moon and back would be 764,342 km.So we need to convert our cm into km…
… oh how fun… this is a metric conversionThis of course is a “2-step conversion” because
both units have a prefix
kmxcmx 1924 10806.11000m
km 1
cm 100
1m 10806.1
I love conversions!
We’re almost done!!!
That’s 23 trillion trips!! Mole-tastic!Marshmallow example: A bed of
marshmallows covering the U.S. would be 776 miles deep
trips1036.2)764,342km
back and trip1( 10806.1 1319 xkmx
Convert moles to # of atoms
How many atoms are in 3.2 mol potassium (K)?Remember: 1 mol = 6.02 x 1023 atomsThis can be written as a conversion factor:
K of atoms x101.9
K of atoms10 x 9.2641 mol 1
10 6.02K mol 2.3
24
23 23
atoms
mol 1
10 6.02 23 atoms
How do we use the “Mole” in chemistry?
The atomic mass of an element is the grams of 1 mole of that atom
Why do chemists use moles?It’s fun.It’s impossible to count atoms with your hands.You can easily measure the mass (in grams) of
a chemical.
Atomic mass = grams of 1 mole of this element, Cobalt
Convert moles of an atom to grams
I need 2.0 moles of copper (Cu) for an experiment. How many grams is that?
Atomic mass of Cu = 63.55 g/mol (round to 2 decimals) “mol” is the abbreviation of “Mole”… I know it’s only
one letter different… chemists!!!
)figs! sig (2Cu 130g
127.01 mol 1
g 55.63Cu mol 0.2
Converting grams to moles
figs) sig (3 mol 2.80
mol 2.79966 g 107.87
mol 1Ag g302
I have 302 grams of silver (Ag). How many moles of silver do I have?Step 1: Atomic mass of Ag = 107.87 g/molStep 2: Calculate
6.2 Molar Mass and Percent Composition
Atomic Mass = mass of one mole of an atom
Molar Mass= mass of one mole of a substance
Calculate Molecular Weights
Example: Calculate the Molecular Weight (MW) of RbI2
Step 1: Assume you have 1 mole of this molecule and determine how much each element weighs from the periodic table.
Step 2: Determine how many of each element you have
Step 3: Add all the masses together
Step 1: Find how much each element weighs from the periodic table
Rb is atomic # 37. How much does each mole of Rb weigh?85.47 grams/mol Rb
I is atomic # 53. How much does it weigh?126.90 g/mol I
Step 2: Determine how many of each element you have
Look at the formula: RbI2
We have 1 “Rb” atom and 2 “I” atoms
Step 3: Add all the masses together
You will need to show this work:
Because the units are the same we can add these two numbers together, so…
253.80 g/mol + 85.47 g/mol = 339.27 g/mol339.27 g/mol is the “molar mass”
g/mol 253.80 I) (2 g/mol) 90.126(
g/mol 85.47 Rb) 1( Rb) g/mol 47.85(
plus
Converting from moles of a compound to grams
Example: I need 3.00 mol NaCl for an experiment. How many grams is that?
Step 1: Find the molar mass
Molar mass = 22.09g/mol + 35.45g/mol
= 57.54 g/mol Step 2: Use the molar mass like a conversion factor.
NaCl 173g
g 62.172mol 1
g 54.57mol 00.3
Converting from grams of a compound to moles
Example: How many moles are in 10.0 g of Na2SO4?Step 1: Find the molar mass.
Molar mass = 142.1 g/mol
Step 2: Use the molar mass like a conversion factor. You need “grams” on the bottom of the fraction.
22 4
1 molmoles of Na SO = 10.0 g 7.04 10 mol
142.1 g
6.3 Formulas of Compounds
Calculate “percent composition”Just like any other %
Stuff = grams of elements
nCompositioPercent 100 x stuff total
stuff
Calculate “percent composition”
Ex: calculate % of Cu and S in Cu2S
Stuff = grams Cu(63.55 g/mol Cu)(2 mol Cu) = 127.1g Cu
Total stuff = grams Cu + grams S
= 127.1 g + 32.07 g = 159.17 g = 159.2 g
nCompositioPercent 100 x stuff total
stuff
%84.79100 x g 159.2
g 127.1
You should be able to…Identify an “empirical formula” and a
“molecular formula”Empirical formula – simplest ratio of atoms of
each element in a compound (whole #’s only)Molecular formula – actual # of atoms of each
element in a compound
Using % composition to determine a formula
Law of Definite Composition – Any amount of a pure compound will always have the same ratio of masses for the elements that make up that compoundEx: H2O is always 88.9% O and 11.1% H by mass
Only the simplest formula (ratio) can be found… in other words, you can only find empirical formulas
Using % composition to calculate the formulaProcess is as follows:
1.Calculate % by mass of each element
2.Determine mass of each element Easy if you use 100 g of the chemical
3.Use mass to find the # of moles of each element
4.Find the smallest ratio of the atoms ÷ the number of moles of each element by the element
with the smallest # of moles Round to the nearest whole #
ExampleA molecule is 75% C & 25% H. Calculate the empirical
formula.Using 100g total = 75g C and 25 g HCalculate moles of each =
Ratio = 6.2C : 25H, simplify by ÷ each by 6.2. Whole number only!!
Final ratio ≈ 1C : 4H so CH4
mol251.01g
mol 1 H g 25 mol 2.6
12.01g
mol 1 C 75g
6.2
25 H
6.2
6.2 C 1 4
C H
12.01g
mol 1 C 75g
1.01g
mol 1 H g 25
6.2
6.26.2
25
C H
Percent 75% 25%
100 g total 75g 25g
Moles
6.2 mol 25 mol
Ratio = =
1 4
Formula CH4
Find the molecular formula
Ex: C3H6O2 is an empirical formula for a chemical. The molar mass of the compound is 148 g/mol.
What is the molecular formula of the compound??
Point: The ratio of C:H:O will always be what the empirical formula shows
Steps1. Calculate the empirical formula mass2. Calculate molar mass/empirical formula mass3. Multiply your subscripts by that #.
Steps1. Calculate the empirical formula mass:
C3H6O2 mass = (3)(12.01) + (6)(1.01) + (2)(16.00) = 148.09 g/mol
2. Calculate (molar mass)/(empirical formula mass) Round to a WHOLE number.
3. Multiply the subscripts of the empirical formula by that number.
C3x2H6x2O2x2 = C6H12O4
274
148