Q how long would it take to spend a mole of 1 coins if they were being spent at a rate of 1 billion per second Background atomic masses Look at the atomic masses on the periodic table What do these represent ID: 491480
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Slide1
The Mole
Q: how long would it take to spend a mole of $1 coins if they were being spent at a rate of 1 billion per second?Slide2
Background: atomic masses
Look at the “atomic masses” on the periodic table. What do these represent?
E.g. the atomic mass of C is 12 (atomic # is 6)
We know there are 6 protons and 6 neutrons
Protons and neutrons have roughly the same mass. So, C weighs 12 u (atomic mass units).
What is the actual mass of a C atom?
Answer: approx. 2 x 10
-23
grams (protons and neutrons each weigh about 1.7 x10
-24
grams)
Two problems
Atomic masses do not convert easily to grams
They can’t be weighed (they are too small)Slide3
The Mole
With
these
problems,
why
use
atomic
mass
at
all?
Masses give information about # of p
+
, n
0
, e
–
It is useful to know relative mass
E.g. Q - What ratio is needed to make H
2
O?
A - 2
:
1 by atoms, but 2
:
16 by mass
It is useful to associate atomic mass with a mass in grams. It has been found that
1
g
H,
12
g
C,
or 23
g
Na
have
6.02
x
10
23
atoms
6.02 x 10
23
is a “mole” or “Avogadro’s number”
“mol” is used in equations, “mole” is used in writing; one gram = 1 g, one mole = 1 mol.
Read 4.3 (167-9). Stop after text beside fig 2.
Do Q1-6. Challenge: 1
st
slide (use reasonable units)Slide4
The Mole: Answers
A mole is a number (like a dozen). Having this number of atoms allows us to easily convert atomic masses to molar masses.
6.02
x
10
23
602 000 000 000 000 000 000 000
3.00 x 6.02
x
10
23
= 18.06
x
10
23
or 1.81
x
10
24
(note: there are 3 moles of atoms in one mole of CO
2
molecules. In other words, there are 5.42
x
10
24
atoms in 3.00 mol CO
2
)
3.01
x
10
23
a) 1.43 kg
12 = 0.119 kg per orange
b) 1.01 g
6.02
x
10
23
= 1.68 x 10
–
24
gSlide5
Mollionaire
Q: how long would it take to spend a mole of $1 coins if they were being spent at a rate of 1 billion per second?A: $ 6.02 x 10
23
/ $1 000 000 000
= 6.02 x 10
14
payments = 6.02 x 10
14
seconds
6.02 x 10
14
seconds / 60 = 1.003 x 10
13
minutes
1.003 x 10
13
minutes / 60 = 1.672 x 10
11
hours
1.672 x 10
11
hours / 24 = 6.968 x 10
9
days
6.968 x 10
9
days / 365.25 = 1.908 x 10
7
years
A: It would take 19 million yearsSlide6
Comparing sugar (C12H
22O11) & H2O
No, sugar has more (45:3 ratio)
Yes (6.02
x
10
23
in each)
Yes.
No, molecules have dif. masses
No, molecules have dif. sizes.
1 mol each
Yes, that’s what grams are.
mass?
No, they have dif. molar masses
# of moles?
No, they have dif. molar masses
# of molecules?
No
# of atoms?
No, they have dif. densities.
volume?
1 gram each
SameSlide7
Molar mass
The mass of one mole is called “molar mass”E.g. 1 mol Li = 6.94 g Li
This is expressed as 6.94 g/mol
What are the following molar masses?
S SO
2
Cu
3
(BO
3
)
2
32.06 g/mol
64.06 g/mol
308.27 g/mol
Calculate molar masses (to 2 decimal places)
CaCl
2
(NH4
)2CO3
O2
Pb3(PO
4)2 C
6H12
O6
Cu x 3 = 63.55 x 3 = 190.65
B x 2 = 10.81 x 2 = 21.62 O x 6 = 16.00 x 6
= 96.00
308.27Slide8
Molar mass
The mass of one mole is called “molar mass”E.g. 1 mol Li = 6.94 g Li
This is expressed as 6.94 g/mol
What are the following molar masses?
S SO
2
Cu
3
(BO
3
)
2
32.06 g/mol
64.06 g/mol
308.27 g/mol
Calculate molar masses (to 2 decimal places)
CaCl
2 (NH
4)2CO3
O2 Pb
3(PO4)2
C6H12O
6
110.98 g/mol (Ca x
1, Cl x
2) 96.11
g/mol (N
x 2, H
x 8, C
x
1, O
x 3)
32.00 g/mol (O
x
2) 811.54 g/mol (Pb
x
3, P x
2, O
x
8)
180.18 g/mol (C
x
6, H
x
12, O
x
6) Slide9
Converting between grams and moles
If we are given the # of grams of a compound we can determine the # of moles, & vise-versa
In order to convert from one to the other you must first calculate molar mass
g = mol x g/mol
mol = g
g/mol
This
can
be
represented
in an
“equation
triangle”
g
mol
g/mol
g= g/mol x mol
0.25
HCl
53.15
H
2
SO
4
3.55
NaCl
1.27
Cu
Equation
mol (n)
g
g/mol
Formula
9.1
36.46
mol= g
g/mol
0.5419
98.08
g= g/mol x mol
207
58.44
mol= g
g/mol
0.0200
63.55Slide10
Simplest and molecular formulae
Consider NaCl (ionic) vs. H
2
O
2
(covalent)
Cl
Na
Na
Cl
Cl
Cl
Na
Na
Chemical formulas are either “simplest” (a.k.a. “empirical”) or “molecular”. Ionic compounds are always expressed as simplest formulas.
Covalent compounds can either be molecular formulas (I.e. H
2
O
2
) or simplest (e.g. HO)
Q - Write simplest formulas for propene (C
3
H
6
),
C
2
H
2
, glucose (C
6
H
12
O
6
), octane (C
8
H
14
)
Q - Identify these as simplest formula, molecular
formula, or both H
2
O, C
4
H
10
, CH, NaCl
H
O
O
H
H
O
O
H
H
O
O
HSlide11
Answers
Q - Write simplest formulas for propene (C
3
H
6
),
C
2
H
2
, glucose (C
6
H
12
O
6), octane (C8
H14)
Q - Identify these as simplest formula, molecular formula, or both H2
O, C4H10
, CH, NaCl
A - CH2
A - H2O is both simplest and molecular
C4H
10 is molecular (C2
H5 would be simplest)
CH is simplest (not molecular since CH can’t form a molecule - recall Lewis diagrams) NaCl is simplest (it’s ionic, thus it doesn’t form molecules; it has no molecular formula)
CH
CH
2O
C4
H7
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