Unit V: The Mole Concept – Atomic Mass, Avogrados Hypothesis, and the Mole (pg , 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 6.02x10 23 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 6.02x10 23 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 = 6.02x10 23 Mg Atoms 1 mole Mg(OH) 2 = 6.02x10 23 Mg(OH) 2 molecules 1 mole O 2 = 6.02x10 23 O 2 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/ ¥ = 6.02 x 10^14 payments = 6.02 x 10^14 seconds 6.02 x 10^14 seconds/60 = x 10^13 minutes x 10^13 minutes/60 = x 10^11 hours x 10^11 hours/24 = x 10^9 days x 19^9 days/ = x 10^7 years It would take 19 million years!
How gases combine Early chemist John Dalton ( ) 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 H 2 reacted with 88.9g O 2 Interpretation O 2 is 8 times heavier (look at PT) 46.7g of N 2 reacted with 53.3g O 2 42.9g C reacted with 57.1g O 2 No real pattern
Joseph Gay-Lussac Combined gas 1L of H 2 reacts with 1L Cl 2 2L of HCl 1L of N 2 reacts with 3L H 2 2L of NH 3 2L of CO reacts with 1L O 2 2L of CO 2 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 N 2 reacts with 3L H 2 2L of NH 3 Lets say each volume contains 1 molecule, we could then say: 1 molecule of N 2 reacts with 3 molecules of H 2 to form 2 molecules of NH 3 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 H 2 reacts with 1L Cl 2 2L of HCl Reactants: 2 hydrogen atoms, 2 Cl atoms Products: 2 hydrogen atoms, 2 Cl atoms 2L of CO reacts with 1L O 2 2L of CO 2 Reactants: 2 carbon atoms, 4 oxygen atoms Products: 2 carbon atoms, 4 oxygen atoms If 2L of H 2 reacts with 1L of O 2, how many litres of H 2 O would be produced? 4 H, 2 O = 2H 2 O = 2L H 2 O Do exercises 2-5 on p. 78
Who can explain this? AAvogadro ’ s Hypothesis EEqual volumes of any gas at standard temperature and pressure contain the same number of molecules This Explains the simple volume ratio for gases
Atomic Mass 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 H 2, O 2, N 2, F 2, Cl 2, Br 2, I 2, At 2 you must remember these Special elements Sometimes Phosphorus is P Sometimes P 4 Sometimes Sulphur is S Sometimes S 8 Assume the rest of the elements are monatomic
Finding the Molar Mass of Compounds H2OH2O = 2(1.0) = 18.0 g/mol Ca(NO 3 ) 2 = (14.0) + 6(16.0) = 164.1g/mol Ammonium phosphate (NH 4 ) 3 PO 4 = 3(14.0) +12(1.0) (16.0) = g/mol HMWK: p80 #6-7