STOICHIOMETRY

Lecture Video Link:

H.W.: 1a-1c, 2a-2c, & 3a-3c

H.W.: WORKSHEET: MOLE TO MOLE - MOLE TO MASS and MASS TO MOLE


WHAT IS STOICHIOMETRY?

Stoichiometry is the quantitative description of the _______________________________ of the substances in a chemical reaction.

When doing experiments, it is unscientific and dangerous to _______________________________ in a haphazard manner. Before any practical laboratory work is done, chemists almost always start with a balanced reaction equation.

This is true for a reaction that we know works, or for one that we predict will work. A _______________________________ helps tell the reaction's stoichiometry.

The stoichiometry of a reaction is the description of the relative quantities by moles of the reactants and products as they appear by the _______________________________ of the balanced equation.

Stoichiometry is the molar bookkeeping of chemistry, and in nature the books must balance.

Quantitative information is only available through its stoichiometry. This applies to the pure study of chemistry, but also to the uses of chemistry in a wide variety of fields such as agriculture, clinical analysis, pharmaceuticals, food chemistry, inhalation therapy, nutrition, forensic science, and the list can go on and on.

The work begins with the writing and _______________________________ of an equation, because this is the work that gives the coefficients - the numbers that disclose the proportiiions by moles.


PROPORTIONAL RELATIONSHIPS: ( a.k.a. MOLE TO MOLE )

We are at last ready to use balanced equations in a practical way to answer some questions about amounts of substances that must be mixed to obtain a given amount of desired product.

It is important to state here that the calculations we will be able to do when we are finished here always present the _______________________________ maximum amounts that must be used or can be obtained.

Recall that:

1. A chemical formula may stand for a single atom or _______________________________ as well as a MOLE of the substance and,

2. The _______________________________ in correctly balanced equations can be used to determine the mass relationships among reactants and products in a chemical reaction.

2 Mg (s) + O2 (g) ® 2 MgO (s)

if the equation were written as:

4 Mg (s) + 2 O2 (g) ® 4 MgO (s)

first, this equation is not reduced, but the ratio remains the same as the first equation.

The Balanced Equation gives the Mole Ratio of _______________________________ Vs. _______________________________.


Example:

How many moles of Potassium Chlorate, KClO3 (s), must decompose in order to produce 9 moles of oxygen gas? The other product is potassium chloride, KCl (s).

- Since we are comparing the number of moles ( ? moles KClO3 : 9 moles O2 ). Recognize that you can use a mole ratio to solve this problem.

FIRST, Write the Balanced Equation.

2 KClO3 (s) ® 2 KCl (s) + 3 O2 (g)

SECOND, From the balanced equation, determine the Mole Ratio of Potassium Chlorate to Oxygen.

The Ratio can be:

or

THIRD, use the mole ratio that will give you moles of Potassium Chlorate, KClO3, in your answer.

FOURTH, Check your answer:

If three times as much Potassium Chlorate decomposes, then this is a proportional relationship - three times as much Oxygen will be produced.


EXAMPLE #1:

Ammonia, NH3, is synthesized from hydrogen gas and nitrogen gas. If 3.8 moles of nitrogen gas reacts with hydrogen gas - how many moles of ammonia would be produced.

 

 

 

 

 

 

EXAMPLE #2:

Glucose. C6H12O6, decomposes into its elemental components (carbon, hydrogen gas, and oxygen gas) in a decomposition reaction. Calculate the number of moles of glucose needed to form 22.4 moles of hydrogen gas.

 

 

 

 

Answers to the Examples:


HOMEWORK PROBLEMS :

1a.) Calculate the number of moles of sodium oxide, Na2O (s), that will be produced when 5.00 moles of solid sodium completely reacts with oxygen gas.

 

 

 

 

1b.) Calculate the number of moles of oxygen gas needed to burn 1.22 moles of ammonia, NH3 (g). The products are nitrogen dioxide, NO2 (g), and gaseous water produced.

 

 

 

 

1c.) Calculate the number of moles of hydrogen gas needed to produce 15 moles of water, H2O.

 

 

Answers to the Homework:


SPECIFYING ACTUAL AMOUNTS: ( a.k.a. MOLE TO MASS or MASS TO MOLE )

The study of the amount of substances consumed and produced in chemical reactions is called stoichiometry. The word is formed from a Greek word stoicheion, meaning "element," and the suffix metry, which means "to measure."

Stoichiometry involves _______________________________ or _______________________________ the amounts of elements or compounds involved in a chemical change. The calculations in stoichiometry problems are not difficult once you recognize what you need to know in order to do the calculation.

The below steps MUST be used for EVERY Stoichiometry question:

1) WRITE A BALANCED EQUATION.
2) Write what you know and then CHANGE THE STARTING QUANTITY TO MOLES.
3) USE THE MOLE RATIO IN THE EQUATION
(obtain from the balanced equation - the coefficients)
4) CONVERT THE NEW MOLES TO THE REQUESTED UNITS. 


EXAMPLE: A typical problem that a Chemical Engineer might face in industry.

Iron metal does not occur in native form to any appreciable extent. Most of it is found in the form of various oxides, iron(III)oxide being fairly common. This oxide is heated in an atmosphere of carbon monoxide to produce iron metal. Carbon dioxide is also a product of this process. How many grams of carbon monoxide are needed to produce 1.0 kg of Iron?

What information do we need to answer this question? Well, we need to know in what proportions Fe and CO react, i.e., a balanced equation:

Fe2O3 + 3 CO ® 2 Fe + 3 CO2

The balanced equation says that 3 moles of CO will produce 2 moles of Fe (assuming you have 1 mole of Fe2O3 ).

But what about 1.0 kg of Fe? Apparently we need moles.

Intuitive arithmetic will now tell us that 27 mol of CO will be required since the ratio between CO and Fe is 3:2. But how does that look on paper?

Finally, since the original question was about grams of CO, we convert the 27 moles to grams:

Above, each step is broken down into its own equation, these equations can be expressed as ONE equation, which would give a MORE accurate answer (eliminating the "Rounding" errors.)


EXAMPLE #3:

A synthesis reaction is performed in the lab where 14.6 moles of potassium react with nitrogen gas to form potassium nitride. Calculate the mass (in grams) of potassium nitride that was created in this reaction.

 

 

 

 

 

EXAMPLE #4:

When 575 grams of methane, CH4, reacts with oxygen gas in a combustion reaction the products of a combustion reaction are formed (you already know what these are for all hydrocarbons). Calculate the number of moles of water vapor created in this reaction.

 

 

 

 

 

Answers to the Examples:


HOMEWORK PROBLEMS:

2a.) How many grams of Potassium Chlorate, KClO3 (s), must decompose to produce potassium chloride, KCl (s) and 1.45 moles of oxygen gas?

 

 

 

 

 

2b.) How many moles of solid copper must react with silver nitrate, AgNO3 (aq), to produce 5.5 grams of solid silver and copper(II) nitrate, Cu(NO3)2 (aq)?

 

 

 

 

 

2c.) Glucose, C6H12O6, is used as a source of energy in the presence of oxygen gas by the human body. The waste products are carbon dioxide gas and water gas. Calculate the number of grams of oxygen gas needed to oxidize 2.5 moles of glucose to carbon dioxide and water.

 

 

 

 

Answers to the Homework:


MASS TO MASS CALCULATIONS:

In the lab, substances are not measured in moles but in units of _______________________________, such as grams.

For example, when working in the lab, there may be times when you will be asked to calculate the number of grams of product yielded by a certain mass of reactant.

You might also have a limited amount of one reactant and must calculate how many grams will be needed to complete a reaction (we will discuss this more when we talk about gases and gas stoichiometry - "Limiting Reactants".)

Although this type of calculation may seem more complex than the calculations you have done so far, it involves no new skills.

The only difference is an additional step to convert the grams of reactant given in the problem to moles.

EXAMPLE: How many grams of oxygen gas are required to react completely with 14.6 grams of solid sodium to form sodium oxide, Na2O (s)?

First, Balance the equation:

4 Na (s) + O2 (g) ® 2 Na2O (s)

Second, Convert the GRAMS to MOLES:

Third, Calculate the MOLES of oxygen from the MOLE RATIO of oxygen to sodium:

Next, Convert MOLES of oxygen to GRAMS of oxygen:

The above steps describe each step to complete the Mass to Mass calculation, but all of these can be done in ONE step:


EXAMPLE #5:

One way to remove CO2 from air that must be recirculated (such as in a spacecraft cabin) is to react it with LiOH to form Li2CO3 and water. How many grams of LiOH are needed for a 6.0 day, 3 person, mission - if in one day, a person exhales about 1.0 kg of CO2? (answer: 2.0 x 104 grams of LiOH)

 

 

 

 

 

EXAMPLE #6:

In combustion reaction where 45 grams of propane gas, C3H8, reacts with oxygen gas to produce the products of a combustion reaction (with a hydrocarbon). Calculate the mass (in kilograms) of carbon dioxide gas created from this reaction.

 

 

 

 

Answers to the Examples:


HOMEWORK PROBLEMS:

3a.) When 20.4 grams of sodium metal are mixed with chlorine gas, how many grams of sodium chloride are produced?

 

 

 

 

3b.) Limestone, calcium carbonate, CaCO3 (s), is heated to produce lime, calcium oxide, CaO (s), and carbon dioxide gas. How much (grams) limestone is required to produce 10.0 kg of lime?

 

 

 

 

3c.) In the decomposition of potassium chlorate, KClO3, 80.5 grams of oxygen gas forms. How many grams of potassium chlorate were present when the reaction started? Also, potassium chloride is produced.

 

 

 

 

Answers to the Homework:

MOLE TO MOLE - MOLE TO MASS and MASS TO MOLE - MASS TO MASS - WORKSHEET


Lecture Video Link:

H.W.: 4a-4d, 5a


MOLARITY AND REPLACEMENT REACTIONS:

The image above shows a piece of copper wire that has been placed in a solution of silver nitrate, AgNO3 (left image frame.) The right frame shows the copper and silver nitrate reaction one day later. The blue solution is characteristic of copper(II) ions.

The assumption is made that copper ions have replaced silver ions in the solution. The metallic silver atoms are attached to the remaining copper wire.

How could one predict how much silver forms when one is dealing with a reactant in solution?

If you know the molarity of the original silver nitrate solution, you can deal with the solution in terms of the number of moles of silver nitrate it contains.


EXAMPLE: How many grams of copper will be required to completely replace silver from 208 mL of a 0.100 M solution of silver nitrate, AgNO3?

First, Write the balanced equation for the reaction:

Cu (s) + 2 AgNO3 (aq) ® Cu(NO3)2 (aq) + 2 Ag (s)

Second, Write down what YOU are being ask to FIND:

208 mL of 0.100 M AgNO3 ® ? grams Cu

Next, Recognize that if you convert the molarity of the solution into moles of silver nitrate, AgNO3, this problem becomes similar to problems you have already solved - converting moles to grams.

Begin by converting molarity to number of moles. Change the volume given in milliliters to liters.

Calculate the number of moles in the solution:

Then calculate the number of moles of copper using the MOLE RATIO of copper to silver nitrate:

From the number of moles of copper - calculate the number of grams of copper to find your final answer.

Again, all these steps can be placed into one equation (which produces a MUCH more accurate answer.)


EXAMPLE #7:

In a single replacement reaction, aluminum metal replaces zinc in a zinc nitrate solution. Calculate the mass (in mg) of zinc metal produced in this reaction when 200 mL of a 0.440 M solution of zinc nitrate is reacted with 0.305 grams of aluminum metal.

 

 

 

 

 

EXAMPLE #8:

325 grams of lead metal is mixed with a 3.45 M solution of iron(III) nitrate. Calculate the volume (in milliliters) of iron(III) nitrate needed to completely react all of the lead metal in this single replacement reaction.

 

 

 

Answers to the Examples:


HOMEWORK PROBLEMS:

4a.) When copper ions replace silver ions in 208 mL of 0.100 M silver nitrate, AgNO3, how many grams of silver will be produced?

 

 

 

 

4b.) When an excess of lead(II)carbonate, PbCO3 (s), reacts with 27.5 mL of 3.00 M nitric acid, HNO3 (aq), what mass (in grams) of lead(II) nitrate, Pb(NO3)2 (aq), will be formed?

 

 

 

4c.) How much (in grams) zinc metal will react with 24.5 mL of 2.0 M hydrochloric acid, HCl?

 

 

 

 

4d.) When an excess of iron(II) chloride reacts with 35.0 mL of 2.5 M potassium sulfide, what mass (in grams) of potassium chloride will be formed? Also, iron(II) sulfide is produced.

 

 

 

 

Answers to the Homework:

MASS TO MASS & MOLARITY TO MASS - WORKSHEET


ADJUSTING TO REALITY:

It is important to remember that a balanced equation can be used to make many predictions. However, there are many factors associated with reactions that are not described by the equation itself.

The equation describes WHAT MIGHT HAPPEN. Balanced equations can be written for reactions that do not occur.

The reality: Some reactions are NOT very _______________________________ and some special conditions are _______________________________.

The ideal chemical reaction involves:

1) Atoms colliding with other atoms
2) Bonds are broken between the atoms (or ions)
3) The "Intermediate" products are formed
4) And Finally, the Products form

In reality, there is usually one REACTANT that will be added in _______________________________ to ensure that the reactant producing the collisions WILL OCCUR with all the atoms from the other reactant (The LIMITING REACTANT).

The LIMITING REACTANT is the reactant that is _______________________________ or the reactant with the least number of moles present.

EXAMPLE:

If 1.21 moles of solid zinc are added to 2.65 moles of hydrochloric acid, HCl, then zinc chloride, ZnCl2 (aq), and hydrogen gas are formed. Determine which reactant is in excess and by what amount and calculate the number of moles of each product.

The questions is asking you to find:

1. The Excess Reactant
2. Amount of Excess
3. The amount of moles of ZnCl2
4. The amount of moles of H2

1) To determine which reactant is in excess, you should use MOLE RATIOS to determine which REACTANT produces the LEAST number of moles of PRODUCT.

( L.R. - Zn)

( EXCESS - HCl )

2) Find number of moles needed to react with limiting reactant ( L.R. )

Amount of Excess: 2.65 mol HCl - 2.42 mol HCl = 0.23 mol HCl of Excess

3) The amount (in moles) of ZnCl2 produced (which DEPENDS on the LIMITING REACTANT.)

4) The amount (in moles) of H2 produced.


HOMEWORK PROBLEMS:

5a.) When 7.24 moles of magnesium, Mg (s), and 3.86 moles of oxygen gas, O2 (g), react to form magnesium oxide, MgO (s), how much product will be produced? (answer: 7.24 mol MgO)

 

 

Answers to the Homework:


ALL STOICHIOMETRY TYPES - WORKSHEET