STOICHIOMETRY

Balance the following equation.

Fe + O2 Fe2O3

4Fe + 3O2 2Fe2O3

This means that when we combine four atoms of iron with three molecules of oxygen gas we will get to molecules of iron oxide.

It also means that if we combine four MOLES of iron with three MOLES of oxygen we can make 2 MOLES iron oxide.

Balance the following equation. C3H8 + O2 CO2 + H2O

C3H8 + 5O2 3CO2 + 4H2O

How many grams of C3H8 do we start with? 44.1 g

How many grams of O2 do we start with? 160g

How much do our products weight? 204.1g

Mole Ratio

We can use the coefficients of balanced chemical equations to set up ratios.

Example: 2K + Br2 2KBr

We can write six mole ratios for this equation.

What are all the possible mole ratios we can write for the following equation?

2KClO3 2KCl + 3O2

Mole Ratios

Determine all possible mole ratios for the following balanced chemical equations.

4Al + 3O2 2Al2O3

3Fe + 4H2O Fe2O4 + 4H2

2HgO + 2Hg + O2

STOICHIOMETRIC CALCULATIONS

Using Stoichiometry

The first step to solving a stoichiometric problem is having a balanced equation.

K + H2O KOH + H2

2K + 2H2O 2KOH + H2

Then we can use mole ratios to solve problems.

If 2 moles of Potassium makes 1 mole of H2 then how many moles of H2 will one mole of Potassium make?

C3H8 + 5O2 3CO2 + 4H2O How many molecules of CO2 are

produced when 10.0 moles of C3H8 is burned with an excess amount of O2?

Practice

Fe + O2 Fe2O3

How many moles of oxygen are required to completely react with 6 moles of iron?

4Fe + 3O2 2Fe2O3

4.5 mol O2

Cu + AgNO3 Cu(NO3)2 + Ag How many moles of Cu(NO3)2 can be produced

from reacting 0.035 moles of AgNO3? Cu + 2AgNO3 Cu(NO3)2 + 2Ag 0.0713 moles of Cu(NO3)2

Mass Calculations

Na + H2O NaOH + H2

How many grams of NaOH can be produced from the reaction of 22.98 grams of Na?

2Na + 2H2O 2NaOH + H2

39.98 g of NaOH How many grams of H2 can be produced

from the reaction of 24 g of H2O? 1.33 g H2

Lab!

The reaction of copper (II) chloride and aluminum is a single replacement reaction.

Al + CuCl3 AlCl3 + Cu 2Al + 3CuCl3 2AlCl3 + 3Cu If our piece of Al initially weights 1.56g

and after the reaction it weights 1.24 g predict the amount of copper in grams that we produced.

WHY DO REACTIONS STOP?

When one of the reactants runs out the reaction will stop

Limiting reactants, like the name says, limits the reaction.

In any reaction the limiting reactant will be completely used up and then the reaction will stop.

Limiting reactants will also limit the amount of product we can make.

Calculations

How can you calculate the amount of products when one reactant is limiting?

S8 + 4Cl2 4S2Cl2 If 200.0 g of S8 reacts with 100.0 g of Cl2

what is the mass of S2Cl2 produced? When the mass of both reactants is given

the first step is to determine which one is the limiting reactant.

S8 + 4Cl2 4S2Cl2 100.0 g Cl2/70.91 (g/mol) Cl2 = 1.410 mol Cl2 200.0 g S8/256.5 (g/mol) S8 = 0.7797 mol S8

The next step is determining which reactant will run out first.

The coefficients from the balanced equation tell us that 4 moles of Cl2 are needed to react with one mole of S8.

If we have 0.7797 moles of S8 how many moles of Cl2 do we need to completely react all of the S8?

4 x 0.7797 = 3.12 mol Cl2 But we only have 1.410 moles of Cl2 so Cl 2 is

our limiting reactant.

16Ag + S8 8Ag2S a.) If 4.00 g of Ag and 4.00 g of S8 react,

which is the limiting reactant? Ag b.) How much of the excess reactant is

left over? 0.0133 mol S8 left over. c.) How many moles of Ag2S will be

produced? 0.0184 moles

2Al + 3I2 2AlI3 a.) If 54.0 g of Al reacts with 50.8 g of I2,

which is the limiting reactant? I2 b.) How much of the excess reactant will

be left over? 0.6 moles Al left over c.) How much product will be produced? 1.4 moles of AlI3 produced

Stoichiometry Tips and Hints

It’s all about the balanced equation and mole ratios.

If one reactant is “in excess” you don’t need to worry about it.

If you’re given moles you can use the mole ratio right away. If you’re given grams you have to convert to moles first.

Think about what the question is asking. When in doubt follow the unit conversion

process. When given amounts of both reactants you must

figure out which is the limiting reactant.

Set problems up and then use calculator mass before = mass after

N2 + 3H2 2NH3

How many moles of NH3 are produced when 25.0 g of N2 react with excess H2?

2KClO3 2KCl + 3O2

If 5.0 g of KClO3 decomposes how many grams of O2 will be produced?

How many grams of KCl will be produced?

Zn + 2HCl ZnCl2 + H2

How many moles of hydrogen are produced when 2.5 g of Zn react with excess hydrochloric acid.

N2 + 3H2 2NH3

How many grams of NH3 can be produced from the reaction between 28 g of N2 and 25 g of H2?

How much of the excess reactant is left over?

Mg + 2HCl MgCl2 + H2

How many grams of hydrogen will be produced when 50 g of Mg react with 75 g of HCl?

How much of the excess reactant is leftover?