I. Stoichiometry · I. Stoichiometry All stoichiometry problems are solved by converting the given units into the units that you are looking for (moles or grams)

I. I. StoichiometryStoichiometryyy

I Stoichiometry = a quantitative study I. Stoichiometry = a quantitative study dealing with relationships between the amounts of reactants and products in a chemical reaction.in a chemical reaction.


Comes from the Greek words stoicheion Comes from the Greek words stoicheion, meaning “element” and metron, meaning “measure”.


Based on chemical equations and the Law Based on chemical equations and the Law Of Conservation Of Matter (matter cannot be either created or destroyed in ordinary chemical or physical means).ordinary chemical or physical means).

Mass of the reactants EQUALS the mass Qof the products!


If you know the amount of one If you know the amount of one substance in a reaction, then you can determine the amounts of all of the other substances.other substances.


All stoichiometry problems are solved All stoichiometry problems are solved by converting the given units into the units that you are looking for (moles or grams). A T-chart is used to obtain or grams). A T chart is used to obtain the correct answer.

II. Mole RatioII. Mole Ratio

What is the role of a mole ratio in What is the role of a mole ratio in reaction stoichiometry problems?

Mole ratio = conversion factor that relates the number of moles of any two substances involved in a chemical reaction.

II. Mole RatioII. Mole Ratio

Obtained directly from the balanced Obtained directly from the balanced chemical equation.

Remember: coefficients in a chemical equation satisfy the law of equation satisfy the law of conservation of matter and represent th l ti b f moles f the relative numbers of moles of reactants and products.

Interpret the following Interpret the following BALANCED equation:BALANCED equation:BALANCED equation:BALANCED equation:

2 Al O (l) 4 Al (s) + 3 O (g)2 Al2O3(l) 4 Al (s) + 3 O2 (g)

2 moles of Al O decompose to 2 moles of Al2O3 decompose to produce 4 moles of Al and 3 moles of O2.

2 Al2 Al22OO33(l) (l) 4 Al (s) + 3 O4 Al (s) + 3 O22 (g)(g)22 33( )( ) ( )( ) 22 (g)(g)

What is the mole ratio between What is the mole ratio between aluminum oxide and aluminum?

2 l Al O 4 l Al2 mol Al2O3 : 4 mol Al

orororor_2 mol Al2O32 3_

4 mol Al

2 Al2 Al22OO33(l) (l) 4 Al (s) + 3 O4 Al (s) + 3 O22 (g)(g)22 33( )( ) ( )( ) 22 (g)(g)

What is the mole ratio between What is the mole ratio between aluminum and oxygen?

4 mol Al3 mol O3 mol O2

2 Al2 Al22OO33(l) (l) 4 Al (s) + 3 O4 Al (s) + 3 O22 (g)(g)

What is the mole ratio between

22 33( )( ) ( )( ) 22 (g)(g)

What is the mole ratio between aluminum oxide and oxygen?

_2 mol Al2O3__3 mol O2

III. Solving III. Solving StoichiometryStoichiometry Problems:Problems:gg yy

Step 1: Write the balanced equationStep 1: Write the balanced equation

Step 2: What do you know? What are you looking for?

SET UP YOUR PROBLEM!!!SET UP YOUR PROBLEM!!!

Step 3: Make a T-chart being sure that Step 3: Make a T-chart being sure that UNITS CANCEL OUT. Always start the T-chart with your given information!chart with your given information!

Conversion Factor to Remember:

1 mole of a substance = molar mass of thatmolar mass of that substance (grams)

Ex 1: MASSEx 1: MASS--MASS Relationship: (MASS Relationship: (givengiven molesmoles) (mole ratio) (unknown ) (mole ratio) (unknown g)g)

Ex 1: MASSEx 1: MASS--MASS Relationship: (MASS Relationship: (givengiven molesmoles) (mole ratio) (unknown ) (mole ratio) (unknown g)g)

Sodium chloride is produced from its elements through a Sodium chloride is produced from its elements through a p gp gsynthesis reaction. What mass of sodium is required to synthesis reaction. What mass of sodium is required to

produce 25 g sodium chloride?produce 25 g sodium chloride?

Balanced Equation=____________________________________________________________

Ex 2: MASSEx 2: MASS--MOLE Relationship: (MOLE Relationship: (givengiven molesmoles) (mole ratio)) (mole ratio)

Ex 2: MASSEx 2: MASS--MOLE Relationship: (MOLE Relationship: (givengiven molesmoles) (mole ratio)) (mole ratio)How many moles of iron (II) bromide would be needed to react How many moles of iron (II) bromide would be needed to react

with potassium phosphate to produce 18.54g of iron (II) with potassium phosphate to produce 18.54g of iron (II) phosphate?phosphate?


Ex 3: How many grams of calcium sulfide would be needed to Ex 3: How many grams of calcium sulfide would be needed to react with 6.32g of silver nitrate to form silver sulfide and react with 6.32g of silver nitrate to form silver sulfide and gg

calcium nitrate?calcium nitrate?


IV. Calculating Percent Yield:IV. Calculating Percent Yield:gg

1) Theoretical yield = maximum 1) Theoretical yield maximum amount of product that can be

d d f i t f produced from a given amount of reactant

a) Theoretical yields are calculated using stoichiometry problems.


2) Actual yield = a measured amount of 2) Actual yield a measured amount of product obtained from a chemical reactionreaction

a) Actual yield must be determined ) yexperimentally or given in a problem.p


3) The actual yield for a product is usually 3) The actual yield for a product is usually LOWER than the theoretical yield for a product. Some reasons include:p

a) Some reactant may be used up in side reactionsside reactions.

b) A product must often be purified ) p pand some product may be lost during purification.


4) Percent yield = the actual yield of 4) Percent yield = the actual yield of a product as a percentage of the theoretical yield


5) Percent yield =5) Percent yield =

_actual yield____ × 100y ____ × 100

theoretical yield

6) Suppose the theoretical yield for the product of a chemical 6) Suppose the theoretical yield for the product of a chemical reaction is 3.67g COreaction is 3.67g CO22 and 3.11 g COand 3.11 g CO22 is actually collected. is actually collected.

Calculate the percent yield for this product.Calculate the percent yield for this product.

7)Suppose the theoretical yield for the product of a chemical 7)Suppose the theoretical yield for the product of a chemical reaction is and 10.7 g SOreaction is and 10.7 g SO2 2 and and 9.49 g SO9.49 g SO2 2 is actually is actually 7 g7 g 2 2 gg 2 2 yycollected. Calculate the percent yield for this productcollected. Calculate the percent yield for this product


8 ) Chemists need to know how efficient a 8.) Chemists need to know how efficient a reaction is in producing the desired product. One way of measuring efficiency is by means of % yield. is by means of % yield.

So ideally we would like the see the % yyield close to 100%!

V. LIMITING & EXCESS REACTANTSV. LIMITING & EXCESS REACTANTS

Limiting Reactant = the reactant that Limiting Reactant = the reactant that limits the amount of the other reactants h bi d h f that can combine, and the amount of products formed in a chemical reaction.

Excess Reactant = the substances that is not used up completely in a reaction.


Each reaction has a limiting reactant. It works the gsame way as in baking. Imagine if you were to go home and mix up a batch of cookie dough to

k f it h l t hi ki d make your favorite chocolate chip cookies and discover that your little brother has been eating the chocolate chips as after-school-snacks and pthere are not enough chips left to make the whole batch of dough into chocolate chip cookies. The amo nt of chocolate chips limits the n mber of amount of chocolate chips limits the number of chocolate chip cookies you can make. You will have to decide what else you can put in the y pbatter to make a different kind of cookie.


In chemical reactions one of the In chemical reactions, one of the reactants will run out before the th thi i th li iti t tother, this is the limiting reactant

because it controls how much product will be made. The reactant that is left over (like the left over cookie dough ( gwithout chocolate chips) is the excess reactant. reactant.


To determine which reactant is limiting and o de e e w c eac a s g a d which is in excess you must do TWO stoichiometry problems. You will be given

f f d two pieces of information and you must use each one to determine the number of moles of product You must solve for molesof product. You must solve for molesbecause it is the common denominator. The reactant that makes the least amount is the limiting reactant and the other is the excess reactant.

Example: If 5.01g of calcium hydroxide react with 2.2g of Example: If 5.01g of calcium hydroxide react with 2.2g of potassium bromide, how many moles of calcium bromide will potassium bromide, how many moles of calcium bromide will

be formed? Identify the limiting and excess reactant. be formed? Identify the limiting and excess reactant.

Step 1:Write the balanced chemical equation.

Ca(OH)2 + 2KBr 2KOH + CaBr2( )2 2



Step 2: You are given two “what you know” items, use each one as a separate problem and solve for the same product. The one that makes the least is limiting and the one that makes the most is excess.

KNOW: 5.01g Ca(OH)2 ? moles CaBr2



5.01g Ca(OH)2_ 1moleCa(OH)2___1 mole CaBr2__ =

74g Ca(OH)2 1 mole Ca(OH)274g Ca(OH)2 1 mole Ca(OH)2

= 0.06 moles CaBr2



KNOW: 2.2g KBr? moles CaBr2



2.2g KBr_ 1mole KBr_1 mole CaBr2__ =

119g KBr 2 mole KBr

= 0.009 moles CaBr2

The KBr is the limiting reactant because it makes the lower number of moles of product.



Step 1:Write the balanced chemical equation.

Ca(OH)2 + 2KBr 2KOH + CaBr2( )2 2

If 6.25g of If 6.25g of HClHCl reacts with 3.4g of NaHCOreacts with 3.4g of NaHCO33, determine how many moles , determine how many moles of sodium chloride are produced. Identify the limiting and excess reactants. of sodium chloride are produced. Identify the limiting and excess reactants.

HClHCl N HCO N HCO N ClN Cl H H O COO COHClHCl + NaHCO+ NaHCO33 NaClNaCl + H+ H22O + COO + CO22

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I. Stoichiometry · I. Stoichiometry All stoichiometry problems are solved by converting the given units into the units that you are looking for (moles or grams)