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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)

- 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

- 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?

- 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

- 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

- 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, 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)

- 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

- 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

- 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!

- 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

- 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

- 11.1g H2 reacted with 88.9g O2
- No real pattern

- Combined gas
- 1L of H2 reacts with 1L Cl2 2L of HCl
- 1L of N2 reacts with 3L H22L of NH3
- 2L of CO reacts with 1L O22L of CO2

- Concluded that gases combine in simple volume ratios
- But why aren’t the volumes of the reactants and products equal?

- Equal volumes of any gas at standard temperature and pressure contain the same number of molecules
- Example:
- 1L of N2 reacts with 3L H22L 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

- 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 O22L 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

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

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

- Some elements are naturally diatomic.
- Remember the “gens”
- Hydrogen, nitrogen, oxygen, halogens

- H2, O2, N2, F2, Cl2, Br2, I2, At2
- you must remember these

- Remember the “gens”
- Special elements
- Sometimes Phosphorus is P
- Sometimes P4

- Sometimes Phosphorus is P

- Sometimes S8

- 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

- (NH4)3PO4

HMWK: p80 #6-7