- In limiting reactions, usually one chemical gets used up before the other.

• The chemical used up first is called limiting reactant
• Once it is used up it stops
• LR determines the quantity of products formed

- To find the LR, assume one reactant is used up. determine how much of this reactant is required.

Other Conversions (Volume and Heat)

• Volume @ STP can be found using the conversion factor 22.4 L/mol.
• Heat can be included as a seperate term in chemical reactions (This is called Enthalpy)
• Rxns that release heat are exothermic.
• Rxns that absorb heat are endothermic.
• Both can be used in stoichiometry.

Mole to Mole

• Mass to Mass Problems involve one additional conversion
• Start from mass A convert to moles A then convert to Moles B to convert to Mass B
• EX. Lead IV Nitrate reacts with 5.0g of Potassium iodide. How many grams of Lead (IV) nitrate are required for a complete reaction?

Pb(NO3)4 + 4KI -> PbI4 + 4 KNO3

5.0g (mass A)   x   mol/166g (Mole A)  x 1/4 (Mole B)  x 455.2/mol (Mass B) = 3.4g

• How many grams of O2 are produced from the decomposition of 3.0g of Potassium Chlorate?

2 KClO3 -> 2 K + Cl2 + 3 O2

3.0g x mol/290.7g x 3/2 x 32/mol = 1.0g

• When Solid Zinc reacts with hydrochloric acid what mass of hydrogen gas is produced when 2.5g of Zinc react?

Zn + 2 HCl -> ZnCl2  + H2

2.5g x mol/65.4 x 1/1 x 2/mol = 0.076g of H2

• If 100g of Octane are burnt in a car engine what mass of oxygen is needed?

2 C8H18 + 25 O2 -> 16 CO2 + 18 H2O

100g x mol/114 x 25/1 x 32/mol = 352 g 

Moles to Mass & Mass to Moles

Map

Mass A to Mol A to Mol B to Mass B

• Some questions will give you the amount of moles and ask to determine the mass
• converting moles to mass only requires on additional step.
• How many grams of Bauxite (Al2 O3) are required to produce 3.5 mol of pure Aluminum?

2 Al2O3 --> 4 Al + 3 O2

3.5 mol x 2/4 x 102/mol = 178.5 = 1.8 x 10^2 g of Al2O3

• How many grams of water are produced of 0.84 mol of Phosphoric Acid is completely neutralized by Barium Hydroxide?

2 H3PO4 + 3 Ba(OH)2 -> 6 HOH + Ba3(PO4)2

0.84 mol x 6/2 x 18/mol = 45.56 = 45 g of H2O

• How many gram of Silver nitrate are needed to produce 1.02 mol of silver chloride according to the reaction.

2 AgNO3 + BaCl2 -> Ba(NO3)2 + 2 AgCl

1.02 mol x 2/2 x 169.9/mol = 173 g

• How many moles of Lead(II) nitrate are consumed when 4.5 g of sodium sulfide completely reacts

Na2S + Pb(NO3)2 -> 2 NaNO3 + PbS

4.5 g x mol/78.1 x 1/1 = 0.058 mols of Pb(NO3)2

Mole to Mole Conversions

• Coefficients in balanced equations tell us the number of moles reacted or produced.
• They can be used as conversion factor

               _X + _Y = _Z

*** WHAT YOU NEED over WHAT YOU HAVE





ex. If 0.15 mol of methane are consumed in a combustion reaction, how many moles of CO2  are produced?

          1CH4 + 2O2 à 1CO2 + 2H2O

          

          0.15mol x 1/1 = 0.15 mol of CO2

ex. How many moles of bauxite (Aluminum Oxide) are required to produce 1.8 mol of pure Aluminum?

          2Al2O3 à 4Al + 3O2

          1.8mol x 2/4 = 0.9mol off Al2O3

ex. When 1.5 mol of copper react with iron(II) chloride. How many moles of iron should be produced?

          1Cu + 1FeCl2 à 1Fe + 1CuCl2

          1.5mol x 1/1 = 1.5mol of Fe