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Unit III: Quantitative

Composition of Compounds

A. Atoms and Isotopes

B. Atomic Composition of Chemical

Compounds

C. Formula and Molecular Mass

D. Calculations using Moles of Atoms

E. Calculations using Moles of Compounds

F. Mass Percent Composition of Compounds

G. Empirical and Molecular Formulas

The Structure of an Atom

nn

npp

p

p

n

e

e

e

e

3-A

What is an atom?

An atom is a fundamental piece of matter, composed of subatomic particles.

An single atom is the smallest amount of an element that maintains properties of that element.

Atoms are not created, changed, or destroyed during a chemical reaction

3-A

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n

n

npp p

p

n

ee

e

e

1.673 x 10-24 g

1.674 x 10-24 g

9.109 x 10-28 g

Electron (-1)

Neutron (0)

Proton (+1)

The Structure of an Atom

3-A

Atomic Mass

Protons and neutrons are regarded as having one atomic mass unit (1 AMU)

Electrons have a mass of zero AMU

The total number of protons and neutrons in an

atom is called the atomic mass

3-A

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Isotopes

While all atoms of the same element behave similarly in chemical reactions, not all atoms

are identical

Atoms always have the same number of protons, but may have different masses….

3-A

N

Isotopes of hydrogen

P

e

H1

1

Atomic mass = 1

H2

1

Atomic mass = 2

H3

1

Atomic mass = 3

HydrogenDeuterium Tritium

N

3-A

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Kinds of isotopes

Stable and radio isotopes

Oxygen for example has isotopes with between 4 and 16 neutrons.

Only those atoms with 8, 9, or 10 neutrons are

stable.

Atomic mass vs. Atomic weight

Atomic Mass refers to the mass of a particular isotope of a particular element

Atomic weight refers to the weighted average mass of all the isotopes of an element.

(you might also see this as relative atomic mass)

UNIT 3-C: Formula and Molecular

weights

Molecular weight refers to covalent compounds

Formula weight refers to the weight of an ionic compound’s formula unit.

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Atomic Composition of Chemical

Compounds

To calculate formula weights we need to know the number of each element in a compound.

3-B

Example: Hexanoic acid CH3(CH2)3CH2COOH

6 carbon atoms

2 oxygen atoms

12 hydrogen atoms

Unit 3-D: Calculations Using Moles of

Atoms

When we talk about practical amounts of elements or compound we are talking about

huge numbers.

We need a convenient term that describes a large group of things (similar to a dozen or

gross). We use the term:

‘mole’

3-D

Moles

A ‘mole’ of anything always contains the same number, 6.022 x 1023

This special number is called Avogadro’s number.

It just happens that there are 6.022 x 1023

atoms of carbon-12 in exactly 12 grams.

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Moles and masses

The atomic mass of C-12 is 12 amu

If the atomic mass of He-4 (for example) is

4 amu.. (1/3 the mass of C-12)

….and there are 12 g of C-12 in one mole of C-12…..

Then there will be 4 g of He-4 in one mole of He-4

Lecture Problem 3-2

A. How many grams are in a mol of

zinc-65 atoms?

B. How many grams are in a mol of all zinc atoms?

Page 90

Lecture Problem 3-3

A. How many grams are in 2.20 mol of iron?

B. Calculate the number of mol in 2.45 x 10-5

g of calcium.

Pg. 90

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Unit 3 Part E:

Calculations using Moles of

Compounds

Calculating molecular weights and

molar masses

If we add up the atomic weights of all the atoms in a compound we get the molecular

weight. (units of amu)

If we want to talk about the mass of 1 mol of a compound we replace the units of amu with

the units g/mol.

Lecture Problem III-4 (pg. 91)

What is the molar mass of ammonium sulfide?

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Unit factors

The concept of the ‘mol’ is very useful in constructing unit factors..

Using the example of glucose (C6H12O6)

With a molar mass of 180.18g/mol

Write as many unit factors as possible utilizing

the mol concept.

Lecture Problem 3-5 (pg. 93)

A. How many mol of hydrogen atoms are in 83.5 g of C6H12O6?

B. How many water molecules are in 0.946 mol of water?

Extra practice: How many total atoms are in 55.3 g of sulfur dioxide?

Using mols to describe

composition of compounds

Lecture problem III-6 (pg. 94)

A sample of ammonium dichromate contains 3.07 mol

of nitrogen atoms. What is the mass of the sample of ammonium dichromate?

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Unit 3-F: Mass Percent Composition

1.Calculate the formula or molecular weight of the compound.

2.Determine the total weight of each element

in the compound.

% composition of an element = Total weight of that element x 100

formula or molecular weight

Unit 3-F: Mass Percent Composition

Lecture problem III-7 (pg. 96).

Calculate the percent by mass of calcium in calcium

phosphate.

Unit 3-G: Empirical and Molecular

Formulas

Given the percent composition of an unknown compound, how can we determine its

identity?

(sometimes actual masses for each element may be given)

The first step is to turn the mass percentage into a ratio of atoms in the compound, called

an empirical formula.

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Unit 3-G: Empirical and Molecular

Formulas

The empirical formula describes the smallest ratio of atoms that are present in a compound.

Consider N2O4 dinitrogen tetraoxide

There is 1 nitrogen atom for every 2 oxygen atoms

So the smallest ratio, or empirical formula) is…

NO2

Solving Empirical Formula Problems

1. If starting with % composition assume exactly 100 grams of sample. Then percentages become mass quantities. (If starting with mass, skip to step 2)

2.Calculate the number of mol of each element present using our unit analysis method.

3.Find the simplest mol ratio by dividing all of the mol quantities by the smallest number of mol of an element present.

Examples

Lecture problems III-8 (page 99)

A.Calculate the empirical formula of a compound that contains 56.4% phosphorus and 43.6% oxygen

B.A compound contains 1.79g of phosphorus and 2.29g of oxygen. What is the empirical formula of the compound?

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Finding the Molecular Formula

Find the empirical formula for the compound.

Calculate the empirical molar mass.

Take the ratio of molecular molar mass to empirical molar mass.

Multiply the quantity of each element in the compound by this factor.

Molecular Formula Determination

Lecture problem (III-9 pg. 101):

a. What is the molecular formula for galactose if the empirical formula is CH2O and the molar mass

is 180.2 g/mol?

b. A molecular compound contains 40.0% carbon, 6.7% hydrogen and 53.3% oxygen by mass.

A 0.100 mol sample of this compound weighs

6.00 grams. What is the molecular formula of the compound?