Stoichiometry

StoichiometryQuantitative AnalysisComposition StoichiometryDeals with mass relationships of elements in compounds

Formula (molar) massConverting grams to moles to atoms/molecules

Find the molar mass of:

Al2O3 =

Al =

O2 =Reaction StoichiometryInvolves the mass relationships between reactants and products in a chemical reaction

Based on balanced chemical equations and the law of conservation of mass

Mole ratio = a conversion factor that relates the amounts in moles of any two substances involved in a chemical rxnRXN Stoichiometry ProblemsAmt (moles) of given amt (moles) of unknown

Ex.

2Al2O3 4Al + 3O2

The coefficients represent the mole ratios of all elements and compounds in a balanced equation.For every 2 moles of Al2O3 , you produce 4 moles of Al and 3 moles of O2EXAMPLE: How many moles of oxygen will be produced if you decompose 5.8 moles of aluminum oxide?RXN Stoichiometry Problems Amt of givenamt of unknownmass of unknown

2Al2O3 4Al + 3O2

EXAMPLE: What was the mass of aluminum oxide decomposed if 5 moles of oxygen is produced?For every 2 moles of Al2O3 , you produce 4 moles of Al and 3 moles of O2RXN Stoichiometry ProblemsMass of givenMoles of givenMoles of unknown2Al2O3 4Al + 3O2If 150 grams of aluminum oxide is decomposed, how many moles of aluminum are produced?RXN Stoichiometry ProblemsMass of givenMoles of givenMoles of unknownMass of unknown2Al2O3 4Al + 3O2If 300 grams of aluminum oxide is decomposed, what is the mass of aluminum produced?You have just calculated the theoretical yield of this reaction.Theoretical v. Actual YieldTheoretical

Amount calculated based on balanced equation and starting amount of reactantActual

Amount actually produced in the lab when the reaction is observed.

Ideal Conditions?Exact required amount of reactants based on equation?

Not realistic

We will run out of supplies of one of the reactants and have excess of the others

Limiting Reagent (reactant)= the reactant that limits the amount of the products that can be produced (because we have a limited amt)

Excess reactant = the substance that is NOT used up completelyDetermining the Limiting ReagentGiven: equation and masses of reactantsUnknown: limiting reactant

Write and balance chemical equationConvert masses (g) of reactants into moles of reactants using their molar massesMultiply mole amount of each reactant by mole ratio to determine mole amount of productsWhichever reactant produces the least of the product is the limiting reactant.Example 1N2H4 (l) + H2O2 (l) N2 (g) + H2O (g)

Which is the limiting reactant when 0.750 mol of N2H4 is mixed with 0.500 moles of H2O2?Example 2Fe (s) + H2O (g) Fe3O4 (s) + H2 (g)

When 36.0 g of water is mixed with 67.0 g of iron, which is the limiting reactant?Calculating Yield (=how much of product is made)After limiting reactant is identified, you can use it to determine theoretical yield

Use mole amount of limiting reactant and mole ratio of equation to determine how much product is produced (just like you did last week)

g givenmol givenmol unknowng unknownExample 1Zn (s) + S8 (s) ZnS (s)

2.00 mol Zn reacts with 1.00 mol of S8

What is the limiting reactant?How many moles of the product are produced (what is the theoretical yield)?C (s) + H2O (g) H2 (g) + CO (g)2.40 mol of C react with 3.10 mol of steam

What is the limiting reactant?Calculate theoretical yield of each productREMINDERSWhen you can calculate the moles used or produced, you can always convert it into grams USING MOLAR MASS!!!

When you are given the mass of something, you MUST FIRST convert it into moles (USING MOLAR MASS!!) before you can begin a problem

Grams Moles Moles Grams