STOICHIOMETRY the study of the quantitative aspects of chemical reactions (the mathematics of chemical reactions)

Why Is It Important? It allows us to predict how much of a reactant is consumed or product made in a chemical reaction It allows us to determine how much reactant will be needed to generate a specified amount of product Very important if a reactant is expensive; using too much wastes money It allows us to determine how much of an unwanted product is generated Very important to minimize this when we’re dealing with medicines Also important if getting rid of the unwanted product is expensive

Essential Questions What is a mole? How do we calculate # of moles from # of atoms and vice versa? Why do we need moles? What is molar mass? How do we calculate the molar mass of an element or a compound?

Let’s relate to something we know What is the relationship between the following words: 1 dozen 1 century 1 decade What do you need to figure out the mass of a dozen eggs if each egg is 10 grams?

Suppose… we invented a new collection unit called a rapp. One rapp contains 8 objects. 1. How many paper clips in 1 rapp? 8 paper clips 2. How many oranges in 2.0 rapp? 16 oranges 3. How many rapps contain 40 gummy bears? 5 rapps

What is a Mole? A) A blind furry animal A brown mark on your body C) A double agent D) An important chemistry concept E) All of the above

Sorry … we are only going to focus on the chemistry concept … The correct answer is E! Sorry … we are only going to focus on the chemistry concept …

A Mole Is Just a Number (a way to count units of something) Just like … A pair = 2 A dozen = 12 A baker’s dozen = 13 A gross = 144 A ream = 500 … a mole = 602,000,000,000,000,000,000,000

Not Just Any Number … 6.02x1023 Avogadro’s Number

Why Avogadro? Named after Amedeo Avogadro who studied quantities of gases and discovered that no matter what the gas was, there were the same number of molecules present at similar conditions This is oversimplified We will revisit it when we study gases

Definition of Avogadro’s Number The number of atoms in exactly 12.00000 grams of the carbon-12 isotope. (nice to know, but not need to know)

Just How Big a Number is a Mole? A mole of M&M’s would cover the continental United States to a depth of 125 km (~77 miles). If the 7 billion people on Earth were to do nothing but count the gumballs in one mole at the rate of one gumball per second, it would take over 2.7 million years to count all the gumballs.

The Most Important Thing to Remember A MOLE IS A NUMBER!

The Mole (mol) is an SI Unit It is the SI unit for the amount of a substance One mole = 6.02 x 1023 particles One mole of eggs = 6.02 x 1023 eggs One mole of cookies = 6.02 X 1023 cookies One mole of cars = 6.02 X 1023 cars One mole of Al atoms = 6.02 X 1023 atoms Notice that the NUMBER is always the same, but the MASS can be very different!

Check for understanding Number of atoms in 0.500 mole of Al a) 500 Al atoms b) 6.02 x 1023 Al atoms c) 3.01 x 1023 Al atoms Number of moles of S in 1.8 x 1024 S atoms a) 1.0 mole S atoms b) 3.0 mole S atoms c) 1.1 x 1048 mole S atoms

Check for Understanding How many atoms are present in 3.7 mol of sodium? 2.2 x 1024 Na atoms How many atoms are present in 155 mol of arsenic? 9.33 x 1025 As atoms How many moles of xenon in 5.66 x 1026 atoms? 940. mol Xe

A Mole of Particles Contains 6.02 x 1023 Particles Regardless of what particles they are 1 mol C = 6.02 x 1023 C atoms 1 mol H2O = 6.02 x 1023 H2O molecules 1 mol NaCl = 6.02 x 1023 NaCl compounds* *technically, ionics are compounds but not molecules, so they are called formula units) *6.02 x 1023 Na+ ions & 6.02 x 1023 Cl– ions

Why are Moles Important? In chemistry, we don’t work with individual atoms or molecules or ions because they are TOO SMALL to be weighed or measured To measure these atoms or molecules or ions, we have to work with LOTS of them That’s where the MOLE comes in 6.02X1023 (a mole) is a practical # of particles to use because this quantity can be weighed more conveniently

Then and Now … THEN When we talked about atoms – we used the atomic mass unit to measure the mass of an atom It is based on the mass of a carbon-12 atom We used the atomic mass from the periodic table as the weight of an atom of an element (in amu)

Then and Now … NOW Recall that the # of atoms in a mole is also based on a carbon-12 atom So, it is reasonable to extrapolate the mass of one atom (in amu) to the mass of one mole of atoms (in grams)

Molar Mass of an Element the mass of 1 mole of an element units = grams/mole numerically equal to the atomic mass (taken from the periodic table) So…the atomic mass from the periodic table can also be used as the mass of one mole!

Molar Mass Example: Elements 1 mole of C atoms = 12.0 g 1 mole of Mg atoms = 24.3 g 1 mole of Cu atoms = 63.5 g (Note: these values are taken directly from the periodic table)

Practice Find the molar mass of the following: 2 moles of Na atoms = 45.98 g 3 mole of Sn atoms = 356.13 g 4 moles of He atoms = 16.0 g

Molar Mass of a Compound (Ionic or Covalent) the mass of 1 mole of a compound units = grams/mole numerically equal to the sum of the atomic masses (taken from the periodic table)

Molar Mass Example: Compounds 1 mole of CaCl2 compound: 1 mole of Ca2+ ions = 40.1 g 2 moles of Cl- ions = 2*35.5 g TOTAL = 40.1 + 71.0 = 111.1 g (Note: these values are taken directly from the periodic table)

Check for Understanding What is the molar mass of N2O4? What is the molar mass of K2O? What is the molar mass of Al(OH)3? What is the molar mass of (NH4)2SO4?

Answers N2O4 = 92.0 grams/mole K2O = 94.2 grams/mole 1 mole of N2O4 = 92g = 6.02x1023 particles K2O = 94.2 grams/mole 1 mole of K2O = 94.2g = 6.02x1023 particles Al(OH)3 = 78.0 grams/mole 1 mole of Al(OH)3 = 78.0g = 6.02x1023 particles

Mole Jokes! Who is Avogadro’s favorite actor? Mol Gibson…. Get it? Ha!Ha! Where did Avogadro go on Saturday? The shopping mole… Wacka!Wacka! What did Avogadro have on his pancakes? Molasses … Why did Avogadro look forward to the year 2000? It was the start of the new molennium …

Stop Work with a partner and practice calculating moles and molar mass on: Try these problems Worksheet: The Mole and Avogadro's number Worksheet 11-2

Why do we use moles? We know that lab balances don’t directly weigh moles Balances weigh in grams We also know that lab balances can’t weigh individual atoms/molecules/ions – or even hundreds or thousands of them These particles are too small to be weighed on balances 31

Why do we use moles? The mole is a link between the microscopic (atoms and molecules) and the macroscopic (grams). We will use the concept of the mole to be able to move between grams and number of atoms or molecules!

Everything must go through Moles!!! Conversions! Everything must go through Moles!!! molar mass Avogadro’s number Grams Moles particles 33

Use molar mass to convert moles to grams Example moles → grams Aluminum is often used for the structure of light-weight bicycle frames. How many grams of Al are in 3.00 moles of Al? Use molar mass to convert moles to grams 34

Use molar mass to convert grams to moles Example grams → moles The artificial sweetener aspartame (Nutra-Sweet) formula C14H18N2O5 is used to sweeten diet foods, coffee and soft drinks. How many moles of aspartame are present in 225 g of aspartame? Use molar mass to convert grams to moles 35

Example How many atoms of K are present in 78.4 g of K? grams → atoms Convert grams to moles Then convert moles to atoms 36

Convert molecules to moles Then convert moles to grams Example molecules → grams What is the mass (in grams) of 1.20 X 1024 molecules of glucose (C6H12O6)? Convert molecules to moles Then convert moles to grams 37

Stop Work with a partner to complete: Try these on your own Moles and Mass Mixed Mole Problems HW: ChemQuest 30