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Unit V: The Mole Concept. 5.1-5.2 – Atomic Mass, Avogrados Hypothesis, and the Mole (pg. 77-85, Hebden ). Today’s Objectives. Explain the significance of the mole, including: Recognize the significance of relative atomic mass, with reference to the periodic table

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Unit V: The Mole Concept

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## Unit V: The Mole Concept

5.1-5.2 – Atomic Mass, Avogrados Hypothesis, and the Mole

(pg. 77-85, Hebden)

### Today’s Objectives

• Explain the significance of the mole, including:

• Recognize the significance of relative atomic mass, with reference to the periodic table

• Identify the mole as the unit for counting atoms, molecules, or ions

• Perform calculations involving the mole, including:

• Determine the molar mass of an element or compound

### The Mole

• Question: how long would it take to spend a mole of 1 Yuan coins if they were being spent at a rate of 1 billion coins per second?

### What is a mole?

• Atoms are REALLY small!

• We can’t work with individual atoms or amu’s (atomic mass units) in the lab

• Why?

• Because we can’t see things that small

### The Mole

• Instead, we work with samples large enough for us to see and weigh on a balance using units of grams

• This creates a problem….

• A pile of atoms big enough for us to see contains billions of atoms!

• Billions of atoms are hard to keep track of in calculations

• So, chemists made up a new unit:

• THE MOLE

### The Mole

• Just as a dozen eggs equals 12 eggs, a mole = 602,000,000,000,000,000,000,000 or 6.02x1023

• It is equal to that number no matter what kind of particles you’re talking about

• It could represent marbles, pencils, or chicken feet

• Usually, the mole deals with atoms and molecules

• The mole, whose abbreviation is mol, is the SI base unit for measuring amount of a pure substance

### The Mole

• The mole, as a unit, is only used to count very small items

• It represents a number of items, so, we can know exactly how many items are in 1 mole

• The experimentally determined number a mole is called is Avogrado’s Number, or 6.02x1023

• The term representative particle refers to the species present in a substance:

• Atoms (most often)

• Molecules

• Formula units (ions)

### Pop Quiz

• 1 dozen Mg atoms =

• 12 Mg atoms

• 1 mole Mg atoms =

• 6.02x1023Mg Atoms

• 1 mole Mg(OH)2 =

• 6.02x1023 Mg(OH)2 molecules

• 1 mole O2 =

• 6.02x1023 O2 molecules

### How big is a Mole?

• 1 Mole of soft drink cans is enough to cover the surface of the earth to a depth of over 320 km

• If you had Avogrado’s number of unpopped popcorn kernels, and spread them across China, the country would be covered in popcorn to a depth of over 15 km

• If we were able to count atoms at the rate of 10 million per second, it would take about 2 billion years to count the atoms in one mole

### Mollionaire

• Back to that question: How long would it take to spend a mole of 1 Yuan coins if they were being spent at a rate of 1 billion per second?

• Answer:

• ¥ 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 19^9 days/365.25 = 1.908 x 10^7 years

• It would take 19 million years!

### How gases combine

• Early chemist John Dalton (1766-1844) wondered how much of a given element would bond (react) with a given amount of another element

• He did not assign an absolute mass for individual atoms of any given element, but rather assigned an arbitrary (relative) mass to each element

• He assumed that hydrogen was the lightest and assigned hydrogen a unit mass of 1

• Through experimentation, he determined that C was 6 times heavier than oxygen, so he assigned C a mass of 6

• Oxygen was found to have a mass 16 times heavier than hydrogen, so he assigned O a mass of 16

• Using this same process, he was able to determine the relative masses of all of the elements

### John Dalton’s Experiment

• Looked at masses of gases

• 11.1g H2 reacted with 88.9g O2

• Interpretation O2 is 8 times heavier (look at PT)

• 46.7g of N2 reacted with 53.3g O2

• 42.9g C reacted with 57.1g O2

• No real pattern

### Joseph Gay-Lussac

• Combined gas

• 1L of H2 reacts with 1L Cl2 2L of HCl

• 1L of N2 reacts with 3L H22L of NH3

• 2L of CO reacts with 1L O22L of CO2

• Concluded that gases combine in simple volume ratios

• But why aren’t the volumes of the reactants and products equal?

### Avogrado’s Hypothesis

• Equal volumes of any gas at standard temperature and pressure contain the same number of molecules

• Example:

• 1L of N2 reacts with 3L H22L of NH3

• Lets say each volume contains 1 molecule, we could then say:

• 1 molecule of N2 reacts with 3 molecules of H2 to form 2 molecules of NH3

• Lets count the atoms to prove this:

• Reactants: 2 nitrogens, 6 hydrogens

• Products: 2 nitrogens, 6 hydrogens

• Mass is always conserved in a chemical reaction, volume is not always conserved in a chemical reaction

### Avogrado’s Hypothesis

• Let’s look at the other 2 examples (again assuming each volume of gas contains 1 molecule):

• 1L of H2 reacts with 1L Cl2 2L of HCl

• Reactants: 2 hydrogen atoms, 2 Cl atoms

• Products: 2 hydrogen atoms, 2 Cl atoms

• 2L of CO reacts with 1L O22L of CO2

• Reactants: 2 carbon atoms, 4 oxygen atoms

• Products: 2 carbon atoms, 4 oxygen atoms

• If 2L of H2 reacts with 1L of O2, how many litres of H2O would be produced?

• 4 H, 2 O = 2H2O = 2L H2O

Do exercises 2-5 on p. 78

Avogadro’s Hypothesis

Equal volumes of any gas at standard temperature and pressure contain the same number of molecules

### Who can explain this?

This Explains the simple volume ratio for gases

The mass of 1 mole of atoms of an element.

The mass of one mole of “C” atoms is 12.0g

The mass of one mole of “Ca” atoms is 40.1g

### Molar Mass (Molecular Mass)

• The mass of 1 mole of molecules of an element or compound

### Diatomic Elements

• Some elements are naturally diatomic.

• Remember the “gens”

• Hydrogen, nitrogen, oxygen, halogens

• H2, O2, N2, F2, Cl2, Br2, I2, At2

• you must remember these

• Special elements

• Sometimes Phosphorus is P

• Sometimes P4

• Sometimes Sulphur is S

• Sometimes S8

• Assume the rest of the elements are monatomic

• ### Finding the Molar Mass of Compounds

• H2O

= 2(1.0) + 16.0 = 18.0 g/mol

• Ca(NO3)2

= 40.1 + 2(14.0) + 6(16.0)

= 164.1g/mol

• Ammonium phosphate

• (NH4)3PO4

= 3(14.0) +12(1.0) + 31.0 + 4(16.0)

= 149.0 g/mol

HMWK: p80 #6-7