The MoleThe Mole Chapter 9

Atomic mass the mass of an atom in atomic mass units (amu) Micro World atomic mass units Macro World grams

Is it useful to use masses in amu? (amu = microscale) NO!! Need to be able to convert to macroscale (grams)

The mole is used to convert from atomic mass units to grams! (amu  g) Why is the mole important??

Discovery of the MOLEDiscovery of the MOLE  Amedeo Avogadro Avogadro’s number (N A ) 6.02 × — is the number of particles in exactly one mole of a pure substance.

How do we normally measure amount?How do we normally measure amount?

mole (mol): amount of a substance that contains as many particles as there are atoms in exactly grams of 12 C 1 mol = 6.02 x Avogadro’s number (N A ) The mole is the SI unit for the amount of substance. 1 dozen = 12

Relating Mass to Numbers of AtomsRelating Mass to Numbers of Atoms Unit: g/mol All molar masses should contain TWO decimal places Molar mass the sum of the atomic masses (in grams) of a compound.

SO 2 1S O+ 2 x SO g/mol 1 molecule SO 2 = amu 1 mole SO 2 = g SO 2

For any molecule formula mass (amu) = molar mass (grams)

Percent CompositionPercent Composition

Percent composition of an element in a compound = n x molar mass of element molar mass of compound x 100% n = the number of moles of the element in 1 mole of the compound (IT IS THE SUBSCRIPT)

Steps to Calculating Percent Composition 1. Calculate total mass 2. Set up ratio of part over whole 3. Multiply by Calculate! All percentages should contain TWO decimal places

H: 2 x 1.01 = 2.02 O: 1 x = g/mol 2 x1.01 g/mol H g/mol H 2 O X 100 = % H g/mol O g/mol H 2 O X 100 = % O Find the percent composition of the elements in water (H 2 O). H : O :

Calculate Total Mass: Al: 2 x = S: 3 x = O: 12 x = g/mol Find the percent composition of the elements in Al 2 (SO 4 ) 3.

Find the percent composition of the elements in Al 2 (SO 4 ) 3. - continued g/mol Al g/mol Al 2 (SO 4 ) 3 X 100 = % Al g/mol O g/mol Al 2 (SO 4 ) 3 X 100 = % O g/mol S g/mol Al 2 (SO 4 ) 3 X 100 = % S Al : S : O :

Mole ConversionsMole Conversions

Converting between unitsConverting between units  The molar mass is used to convert the mass of a substance in grams to moles and the number of moles to grams of a substance.

11.2 g NaCl Mass to Mole Conversions 1 mol NaCl g NaCl mol NaCl Na: 1 X = Cl: 1 X = g/mol ÷ MOLAR MASS

3.2 mol Zn(NO 3 ) 2 Mole to Mass Conversions g Zn(NO 3 ) 2 1 mol Zn(NO 3 ) g Zn(NO 3 ) 2 Zn: 1 X = N: 2 X = O: 6 X = g/mol x MOLAR MASS

Stoichiometry MOLE

Mole RatioMole Ratio  mole ratio : a conversion factor that relates the amounts in moles of any two substances involved in a chemical reaction ALWAYS NEED A BALANCED CHEMICAL EQUATION!!! Example: 2Al 2 O 3(l) → 4Al (s) + 3O 2(g) Mole Ratios: 2 mol Al 2 O 3 2 mol Al 2 O 3 4 mol Al 4 mol Al 3 mol O 2 3 mol O 2

Steps for completing stoichiometry:Steps for completing stoichiometry: 1. Write a balanced chemical equation. 2. Identify what is given. 3. Identify what is being found. 4. Write what is given. 5. Write the mole ratio. 6. Calculate unknown.

Sample problem: How many moles of HCl are needed to react with 2.3 moles Zn? Eqn: __Zn + __HCl  __ZnCl 2 + __H 2 Given: Unknown: =

Mole to Mole How many moles of hydrogen are produced when you start with 3.0 moles of NH 3 ? Given: Unknown: __NH 3  __N 2 + __H mol NH 3 3 mol H 2 2 mol NH 3 = 4.5 mol H 2

DO NOWDO NOW Balance the following equation: ___Fe + ___O 2  ___Fe 2 O 3 If you start with 3.23 moles of Fe, how much Fe 2 O 3 will be produced?

Limiting ReactantsLimiting Reactants

 limiting reactant : the reactant that limits the amount of product that can form  excess reactant : the substance that is not used up completely in a reaction.

You are at home and ready to make some Betty Crocker Brownies to bring to chemistry class. You know the class is big so you need to make two batches of brownies. You gather all the supplies and this is what you have: 2 Packages of Brownie Mix 1 cup of water 1 cup of vegetable oil 3 eggs Do you have enough supplies to make the brownies? What is limiting you? What is in excess? Ingredients on Brownie Box: 1 Package of Brownie Mix ¼ cup of water ½ cup of vegetable oil 2 eggs

Percent YieldPercent Yield

 expected yield: maximum amount of product that can be produced from a given amount of reactant.  actual yield: measured amount of that product obtained from a reaction.  percent yield: ratio of the actual yield to the expected yield, multiplied by 100. expected yield

Percent Yield ExamplePercent Yield Example In an experiment, the scientist expected to produce 16.0 grams of silver in a reaction. The actual yield was 15.2 grams of silver. What is the % yield? Cu + 2 AgNO 3  Cu(NO 3 ) Ag