Chapter 7: Chemical Quantities
Learning Objectives:
1) Use atomic mass from periodic table to calculate molecular mass of a molecule
2) Understand the relationship between the “amu” and the “mole”
3) Be able to convert between grams and moles using molecular mass
4) Be able to convert between moles and # particles using Avogadro's number
5) Differentiate between empirical and molecular formula
6) Be able to determine the empirical formula given experimentally determined mass
percentage values for a given substance
7) Be able to compute the molecular formula given objective (6) and the experimentally
determined molecular mass for a given substance
Molecular Mass
„ Atoms have an atomic mass. Shouldn’t a molecule have a
molecular mass? Of course. Determining the molecular
mass is as easy as it should be.
„
H has an atomic mass of 1.00794 amu
H2 has a molecular mass of 2(1.00794amu) = 2.01588 amu
„
NaCl has a MM of
„
22.99 amu
+ 35.45 amu
58.44 amu
Some Atomic Masses
C
12.01 amu
N
14.01 amu
O
16.00 amu
F
19.00 amu
S
32.065 amu
Cl
35.45 amu
Na
22.99 amu
Ca
40.08 amu
„ Calculate the molecular mass for the following
a) Sulfur dioxide
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b) ammonia
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c) Calcium chlorate
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Expressing Large Values
„ In every day life, we often give names to a certain number of things
„ Ten years is called a decade
„ Twelve donuts is a dozen donuts
„ A dozen dozen of eggs (144 eggs) is called a gross of eggs
„ When dealing with these ‘package’ of things, we usually get information in
the form of “so many of these per number of things”. For example:
„
„
„
Number of solar flares per decade
15 slices of bread per loaf
Mass per dozen eggs
„ “how much does a dozen eggs weigh? 45g per dozen.”
„ Assume that a dozen, grade 3 eggs have a mass of 750.00g
„ What is the mass of 6.5 dozen, grade 3 eggs?
„
How many dozen grade 3 eggs are in a container holding a mass of 10312.3g?
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What’s In A Gram?
„ OK, so we now have atomic and molecular mass values for
any substance with a known formula
„ Question, what is an amu?
„
We learned its value in ch 3 as 1 amu = 1.66054 × 10-24 g
„ Problem is, no one has an instinct for this size mass.
„ We live in a macroscopic world with masses usually larger
than 1 gram, and certainly larger than milligrams
„ It might be better to develop a manner to speak of the AM and
MM in more ‘normal’ sized values
„ In the end, we’ll always talk about a bunch of atoms or
molecules whenever we are obtaining their mass
„ We won’t weigh up one, but a whole lot of them. How
many?
„ Suppose a particle has a mass of 1 amu. How many of
these particles do we need to have 1 gram total mass?
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The Mole
„ This special number of particles, 6.022137 × 1023 has been
given a special name: the mole
„
1 mole of anything contains 6.022×1023 of those things
„ This number is also called Avogadro's number (symbol N0)
„ The mole is used exactly in the same manner as the dozen
„ Both represent a certain number of objects, no matter what
kind of object you are talking about.
„ 1 dozen = 12 things
„ 1 mole
= 6.022×1023 things
„ Assume that a mole of space dust has a mass of 750.00g
„ What is the mass of 6.5 moles of space dust?
„
How many moles of space dust in a container holding a mass of 10312.3g?
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Mole Within Molecules
„ A mole contains a bunch of molecules (it’s unimaginable)
„ Also, a molecule can contain a bunch of atoms
„ For example, ammonium phosphate: (NH4)3PO4
„ Here, one molecule has 3 N atoms, 12 H atoms, 1 P atom and 4 O
atoms.
„ You have 1.5 moles of tricycles.
a. How many moles of seats do you have?
b. How many moles of tires do you have?
c. How could you use parts a and b as an analogy to teach a friend about
the number of moles of OH- ions that are in 1.5 moles of Mg(OH)2?
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Repackaging the Periodic Table
„ We chose the value of the mole so that the periodic table’s
values were in a ‘grams per mole’ ratio
„
„
„
The molecular mass of H2 is 2.016 grams per mole
This represents a normal sized number, good for all of us
It does not represent the mass of 1 H2, but a mole of H2!
„ Imagine you have a sample of sodium chloride with a mass of 1052g.
„ Since MM of NaCl is 58.44 amu, we know that 1 mole NaCl = 58.44 g
„ How many moles of sodium chloride do we have, in our sample?
„
How many molecules of sodium chloride do we have, in our sample?
„
How many sodium atoms do we have, in our sample.
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Concept Check
„ What is the mass of 1 mole of NaOH?
„ How much does 106 He atoms weigh?
„ I have 12.0 oz. Gold. How many moles is that?
„ 16 oz = 1 lb, 1 lb = 453.6 g
„ Finally, how many gold atoms are in this 12.0 oz?
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Composition Concepts
„ The formula of a compound contains much information
„ Try NaOH. We’re told that sodium hydroxide contains an equal
number of sodium, oxygen, and hydrogen atoms.
„ So, there are equal moles of sodium, oxygen, and hydrogen
atoms within a sample of sodium hydroxide.
„ Suppose a sample of sodium hydroxide has a mass of 60g.
„ Does the above concept mean that there is 20g of Na,
20g of O, and 20g of H in the sample?
„ It is important to note at this time that the reason we
measure the property of matter called mass is to count the
number of particles (atoms, ions or molecules).
„
Since different particles have different masses, shouldn’t equal
numbers of different particles have a different mass?
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Percent By Mass
„
„
There are many methods to determine the mass of each type
of atom in a molecule. So, it is relatively easy to measure the
percent by mass of each atom type in a molecule.
„
By definition,
AMof atom × # of these atoms
× 100
the percent mass is %mass atom in molecule =
MM
„
For sodium in sodium hydroxide
%mass Na in NaOH =
22.99
× 100 = 57.48% Na
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In your group, try a couple:
„
The % mass of C in methane is…
i. 20.00 %
ii. 74.83 %
iii. 92.24 %
„
The % mass of H in water is…
i. 5.605 %
ii. 5.938 %
iii. 11.21 %
iv. 66.67 %
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Empirical vs. Molecular Formula
„
Fill in the remainder of the table below. A few cells have been completed to aid you.
Name
Molecular Molecular Formula Mass (amu)
Empirical Empirical Formula Mass (amu)
Ethene
C2H4
Propene
C3H6
Butene
C4H8
CH2
Acetylene
C2H2
CH
Benzene
C6H6
MM/EM
% mass C
% mass H
14.40
4
13.02
92.24
78.12
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Questions Based on the Above Table
„ Define empirical formula based upon what you see in the table?
„ Is it possible to determine the molecular formula for a compound based
solely upon the percent mass? Explain your reasoning.
„ What column of the table clearly differentiates the molecular formula from
the empirical formula for a compound? Justify your response.
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Empirical Formula Example
„ Water (11.21% H and 88.79% O)
„ Assume 100 g sample so that we have
# g H = (0.1121)(100.g ) = 11.21g H
# g O = (0.8879)(100.g ) = 88.79g O
„
„
Find number of moles of each atom in compound
# mol H = 11.21g H ⋅
mol H
= 11.12mol H
1.008g H
# mol O = 88.79g O ⋅
mol O
= 5.549mol O
16.00 g H
Write the formula for this compound (mole ratio)
H 11.12O 5.549
„
Divide through by smallest mole amount
H 11.12O 5.549 ⇒ H 2.004O ⇒ H 2O
5.549
„
5.549
We know the last step is true because the atoms must come
in whole numbers, and the extra amount (0.004) is just
experimental error or an error in the purity of our sample
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An Empirical Formula Problem
„
An experiment was completed on an unknown sample from a
local mineral spring. A contaminant was analyzed by
composition to give the data to the right. What is the empirical
formula for the unknown contaminant?
„
To find the molecular formula, what additional data would you need?
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Atom
% mass
H
3.74%
C
37.4%
Cl
58.9%
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Another form of the Problem
„
A binary compound is composed of lithium and oxygen. When the compound is
heated to extreme temperatures, the compound will decompose into lithium atoms and
oxygen molecules.
„
In one experiment, 10.00 grams of the binary compound decomposed, leaving a pile of lithium
4,464g. What is the empirical formula of the binary compound?
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Getting the Molecular Formula
„ There are many methods of determining the molecular mass of a compound
„ Experimentation must be done
„ If we know the MW, then the molecular formula is easy to determine
„ The empirical formula for a compound is C2H3O and its molecular mass is
86.10 amu. What is the molecular formula for this compound?
„
C2H3O has a formula weight of 43.05 amu
The empirical formula is some multiple off of the real
molecular formula (n = 1, 2, 3 …)
Thus, the relationship for the formula is (C 2H3O)n =Molecular Formula
„
And the relationship for the formula is FW ⋅ n = MW
„
So the molecular formula is always Molecular Formula = (C2H3O) MW
„
„
FW
„ Thus, our sample has the molecular formula
(C2H3O) 86.1 = (C 2H3O)2 = C 4H6 O2
43.05
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Concept Check 2
„ A sample of acetaldehyde is 54.5% C, 9.20% H and 36.3% O, and has a
molecular mass of 44 amu. What is acetaldehyde’s molecular formula?
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