Transcript 10.2The Mole-Mass Relationship
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? WwWWwwhhH
HHhhyyYY
YYyy???
6.1 Atoms and Moles Amedeo Avogadro
Avogadro’s Number
6.022 x 10 23
Avogadro discovered that there are 6.022 x 10 23 atoms in 1 gram of hydrogen.
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”. Dozen means “12”. You can have a dozen of anything. You can also have a mole of anything.
Hmmm… I shall call 6.022 x 10 to it.
23 ... a “mole”. Yes…that has a nice ring
So How Big is a “MOLE”
Ummm… NO!
Here it is written out 602,200,000,000,000,000,000,000 That’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?
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: 3
cm clip x
6 .
02
x
10 23 1
mole clips
1 .
806
x
10 24
cm mole
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 conversion
This of course is a “2-step conversion” because both units have a prefix
1 .
806
x
10 24
cm
1m 100 cm 1 km 1000m 1 .
806
x
10 19
km
I love conversions!
We’re almost done!!!
1 .
806
x
10
19
km
( 1 trip and back 764,342km )
2 .
36
x
10
13
trips
That’s 23 trillion trips!! Mole-tastic!
Marshmallow example: A bed of marshmallows covering the U.S. would be 776 miles deep
Convert moles to # of atoms
How many atoms are in 3.2 mol potassium (K)?
Remember: 1 mol = 6.02 x 10 23 atoms This can be written as a conversion factor: 6.02
10 23 1 mol
atoms
3 .
2 mol K 6.02
10 23 1 mol
atoms
1 9.264
x 10 23 atoms of K 1.9
x10 24 atoms of K
How do we use the “Mole” in chemistry?
The atomic mass
grams of 1 mole
of an element is the 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!!!
2 .
0 mol Cu
63 1 .
55 mol g
127.01
130g Cu (2 sig figs!
)
Converting grams to moles
I have 302 grams of silver (Ag). How many moles of silver do I have?
Step 1: Atomic mass of Ag = 107.87 g/mol
Step 2: Calculate
302 g Ag
1 mol 107.87
g
2.79966
mol
2.80
mol (3 sig figs)
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 RbI 2
Step 1 Step 2 Step 3 : Assume you have 1 mole of this molecule and determine how much each element weighs from the periodic table.
: Determine how many of each element you have : 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: RbI 2 We have 1 “Rb” atom and 2 “I” atoms
Step 3: Add all the masses together You will need to show this work:
( 85 .
47 g/mol Rb) ( 1 Rb) 85.47
g/mol
plus
( 126 .
90 g/mol) (2 I) 253.80
g/mol
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/mol 339.27 g/mol is the “ molar mass ”
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.
3 .
00 mol
57 1 .
54 mol g
172 .
62 g
173g NaCl
Converting from grams of a compound to moles
Example: How many moles are in 10.0 g of Na 2 SO 4 ?
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.
moles of Na SO = 10.0 g 2 4 1 mol 142.1 g 2
6.3 Formulas of Compounds
Calculate
“percent composition”
Just like any other % stuff total stuff x 100 Percent Stuff = grams of elements Compositio n
Calculate
“percent composition”
Ex: calculate % of Cu and S in Cu 2 S
stuff total stuff x 100 Percent Compositio n
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
127.1
159.2
g g
= 159.17 g = 159.2 g
x 100
79 .
84 %
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
Molecular
H 2 O 2 C 3 H 6 N 2 O 3 C 2 H 6 C 3 H 9
Empirical
HO CH 2 N 2 O 3 ?
?
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 compound Ex: H 2 O 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 formula
Process 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 #
Example
A molecule is 75% C & 25% H. Calculate the empirical formula.
Using 100g total = 75g C and 25 g H Calculate moles of each = 75g C 1 mol 12.01g
C
6 .
2 mol 25 g H 1 mol 1.01g
H
25
mol
Ratio = 6.2C : 25H, simplify by ÷ each by 6.2. Whole number only!! C 6.2
6.2
H 25 6.2
Final ratio ≈ 1C : 4H so CH 4
4
Percent 100 g total Moles Ratio Formula C
75% 75g 75g C 1 mol 12.01g
6.2 mol 6.2
6.2
= 1 CH 4
H
25% 25g 25 g H 1 mol 1.01g
25 mol 25 6.2
= 4
Find the molecular formula
Ex: C 3 H 6 O 2 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
Steps
1. Calculate the empirical formula mass
2. Calculate molar mass/empirical formula mass
3. Multiply your subscripts by that #.
Steps
1. Calculate the empirical formula mass: C 3 H 6 O 2 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.
148 74 2
3. Multiply the subscripts of the empirical formula by that number.
C 3x2 H 6x2 O 2x2 = C 6 H 12 O 4