Stoich Notes

8/14/2019 Stoich Notes

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Gravimetric StoichiometryGravimetric StoichiometryBy prof Devender Singh RoorkeeBy prof Devender Singh Roorkee

Notes and ExamplesNotes and Examples


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IntroductionIntroduction

What is stoich-i-o-metry?What is stoich-i-o-metry? It is the methods used to calculate the quantities ofIt is the methods used to calculate the quantities of

substances in a chemical reaction.substances in a chemical reaction. ** Gravimetric (mass of solids) **** Gravimetric (mass of solids) **

Solution (volume and concentration of solutes)Solution (volume and concentration of solutes) Gas (volume, pressure and temperature of gases)Gas (volume, pressure and temperature of gases)

What do you need to know?What do you need to know? good problem-solving skillsgood problem-solving skills experience with lab procedures and techniquesexperience with lab procedures and techniques good background chemistry knowledge (bothgood background chemistry knowledge (both

theoretical and empirical)theoretical and empirical)


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ReviewReview

You must have a solid understanding of the following toYou must have a solid understanding of the following toachieve success in this unit:achieve success in this unit: The MoleThe Mole Molar MassMolar Mass

how to calculate molar mass of elements and compounds usinghow to calculate molar mass of elements and compounds using

values found on the periodic tablevalues found on the periodic table The Mass formulaThe Mass formula

m = nM, n = m/M, M = m/nm = nM, n = m/M, M = m/n

know how to rearrange formula for any variable requiredknow how to rearrange formula for any variable required

Writing Correct Chemical FormulasWriting Correct Chemical Formulas

for all elements and compounds, including their statesfor all elements and compounds, including their states

Balancing Chemical EquationsBalancing Chemical Equations this is the first step in every stoichiometry problem and thereforethis is the first step in every stoichiometry problem and therefore

mastery of this skill is vital to answering a question correctly.mastery of this skill is vital to answering a question correctly.


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Significant DigitsSignificant Digits

this is to ensure common communication and accuracy inthis is to ensure common communication and accuracy inall calculated and measured values in chemistryall calculated and measured values in chemistry

we will use the common rules used on the diploma examwe will use the common rules used on the diploma examin Chemistry 30in Chemistry 30

Here are a few of the basics:Here are a few of the basics: your answer cannot be more accurate than the values given in theyour answer cannot be more accurate than the values given in thequestion (use thequestion (use the leastleast number of sig digs from the question).number of sig digs from the question).

Sig digs include all digits correctly reported from a measurement,Sig digs include all digits correctly reported from a measurement,except leading zerosexcept leading zeros

exact values have an unlimited number of sig digsexact values have an unlimited number of sig digs

When adding values, use the least number of decimal places found inWhen adding values, use the least number of decimal places found in

the measurementsthe measurements molar mass is recorded to 2 decimal placesmolar mass is recorded to 2 decimal places you may need to use scientific notation to write correct number ofyou may need to use scientific notation to write correct number of

significant digits.significant digits.


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Stoichiometry Concept MapStoichiometry Concept Map

Stoichiometry


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Stoichiometry ProcedureStoichiometry Procedure

Balanced Chemical Equation

Given quantity, findmoles of given

(mass, volume, concentration)

From moles calculated,find quantity of required

(mass, volume, concentration)

From moles calculated,

find moles of required

(use mole ratio formula)


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1.1. Write a balanced equation for the reaction.Write a balanced equation for the reaction.2.2. Determine the two substances involved in theDetermine the two substances involved in the

reaction, the given and the unknown.reaction, the given and the unknown.

3.3. *Calculate the number of moles of the given*Calculate the number of moles of the given

substance.substance.4.4. Set up the mole ratio using coefficients of theSet up the mole ratio using coefficients of the

balanced equation for the two substances.balanced equation for the two substances.

5.5. Substitute values in mole ratio for the givenSubstitute values in mole ratio for the given

and the unknown.and the unknown.6.6. Solve the equation for the unknown number ofSolve the equation for the unknown number ofmoles.moles.

7.7. *Convert required moles into mass by*Convert required moles into mass bymultiplying the unknowns molar mass.multiplying the unknowns molar mass.

Stoichiometry Steps


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Note:Note: You may not always use steps 3 and 7 (only in 2 or 3 step stoichYou may not always use steps 3 and 7 (only in 2 or 3 step stoich

problems)problems) A correct balanced equation is vital for full marks as it effects allA correct balanced equation is vital for full marks as it effects all

other steps and calculationsother steps and calculations

Always list the given and unknown variables under eachAlways list the given and unknown variables under eachsubstance in your chemical equation for step 2.substance in your chemical equation for step 2.

Check for correct significant digits and include all units whenCheck for correct significant digits and include all units whenrecording your final answer in a boxrecording your final answer in a box

Marks distributed as follows on any stoich question:Marks distributed as follows on any stoich question: 1 mark for balanced chemical equation1 mark for balanced chemical equation 1 mark for each step of the procedure1 mark for each step of the procedure 2 marks for final answer (1 mark for value, for units, for correct2 marks for final answer (1 mark for value, for units, for correct

significant digits)significant digits)

Show ALL your work for full marks!!!Show ALL your work for full marks!!!


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1 step problems: Mole to Mole1 step problems: Mole to Mole

The Mole RatioThe Mole Ratio A mathematical relationship of theA mathematical relationship of the# of moles between the given and# of moles between the given and

the unknown found in thethe unknown found in the

balanced chemical equation.balanced chemical equation.

Example:Example:Given the following equation, writeGiven the following equation, write

the molar ratios for:the molar ratios for:

a)a)

c)c)

b)b)

G

G

C

Cnn =

)(3)(2)(2 23 ggg NHHN +

=

2

2

H

N

=

3

2

NH

N=

2

3

H

NH


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Example #1Example #1 During the production of ammonia,During the production of ammonia,

If 3.00 moles of HIf 3.00 moles of H22 were used, how many moles of NHwere used, how many moles of NH33 would be made?would be made?

If 0.600 moles of NHIf 0.600 moles of NH33 were produced, how many moles of Nwere produced, how many moles of N22 arearerequired?required?


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More Examples and PracticeMore Examples and Practice

Go through examples on p.160 of yourGo through examples on p.160 of yourworkbook.workbook.

Do Practice Problems #1-9 on p.160-164Do Practice Problems #1-9 on p.160-164of workbook (random HW check showof workbook (random HW check showall your work)all your work)

Work on Stoichiometry Assignment #1Work on Stoichiometry Assignment #1 due Thursday, November 2due Thursday, November 2ndnd


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2 step problems:2 step problems:Mole to Quantity or Quantity to MoleMole to Quantity or Quantity to Mole

An additional step is neededAn additional step is needed to find the # of moles of given, orto find the # of moles of given, or to find the mass of the unknown.to find the mass of the unknown.

Need to use the mass formula, m=nM, to solve forNeed to use the mass formula, m=nM, to solve foreither neither ngivengiven or mor munknownunknown

May use unit conversion for this step if desiredMay use unit conversion for this step if desired


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Example #2Example #2 During the combustion of methane,During the combustion of methane,

how many grams of carbon dioxide are produced if 2.0 moles of methanehow many grams of carbon dioxide are produced if 2.0 moles of methaneare used?are used?

how many moles of methane are needed to produce 12.2 grams of waterhow many moles of methane are needed to produce 12.2 grams of watervapour?vapour?


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Go through examples on p.165-166 of yourGo through examples on p.165-166 of yourworkbook.workbook.

Do Practice Problems #1, 6, 8, and 9 on p.166-Do Practice Problems #1, 6, 8, and 9 on p.166-

167 of workbook (random HW check)167 of workbook (random HW check)

Finish Stoichiometry Assignment #1Finish Stoichiometry Assignment #1 due Thursday, November 2due Thursday, November 2ndnd

Go through example #2 on p.169 and doGo through example #2 on p.169 and doPractice Problems #1-4, 6, 8-11 on p.169-171 ofPractice Problems #1-4, 6, 8-11 on p.169-171 ofworkbookworkbook


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3 step Problems:3 step Problems:Quantity to QuantityQuantity to Quantity

In this type of question, you are providedIn this type of question, you are providedwith a quantity of one substance, and askedwith a quantity of one substance, and askedto calculate a quantity such as mass ofto calculate a quantity such as mass of

another substance.another substance. All 7 stoichiometry steps are needed andAll 7 stoichiometry steps are needed and

each question is worth 6 markseach question is worth 6 marks Note: these are never asked in a multiple choiceNote: these are never asked in a multiple choice

questionquestion Show ALL your work in each stepShow ALL your work in each step


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Example #3Example #3 During a reaction between lithium hydroxide and hydrobromic acid, if youDuring a reaction between lithium hydroxide and hydrobromic acid, if you

start with 3.7 grams of lithium hydroxide, how many grams of lithiumstart with 3.7 grams of lithium hydroxide, how many grams of lithiumbromide will be produced?bromide will be produced?


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Example #4Example #4 If you start with lead (II) nitrate and 15.0 grams of sodium iodide, how many gramsIf you start with lead (II) nitrate and 15.0 grams of sodium iodide, how many grams

of sodium nitrate can be formed?of sodium nitrate can be formed?


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Go through example #1 on p.172 of yourGo through example #1 on p.172 of yourworkbook.workbook.

Do Practice Problems #3, 5, 6, and 9-11 onDo Practice Problems #3, 5, 6, and 9-11 onp.173-175 of workbook (random HW check)p.173-175 of workbook (random HW check)

Finish Stoichiometry Assignment #2Finish Stoichiometry Assignment #2 due Wednesday, November 8due Wednesday, November 8 thth

Stoich Quiz on Thursday, Nov. 9Stoich Quiz on Thursday, Nov. 9 thth


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Limiting ReagantsLimiting Reagants

During a chemical reaction, one reactantDuring a chemical reaction, one reactantwill usually run out first. This substance iswill usually run out first. This substance iscalled the limiting reagant and stops thecalled the limiting reagant and stops the

formation of new products.formation of new products. An additional step is needed to compareAn additional step is needed to compare

the moles of both reactants to find whichthe moles of both reactants to find whichreagent will be used up firstreagent will be used up first usually the smallest mole amount but beusually the smallest mole amount but be

careful and check the mole ratio amountscareful and check the mole ratio amounts


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Example #5Example #5

If you start with 25.0 grams of lead (II) nitrate and 15.0 grams of sodiumIf you start with 25.0 grams of lead (II) nitrate and 15.0 grams of sodiumiodide, how many grams of sodium nitrate can be formed?iodide, how many grams of sodium nitrate can be formed?


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Example #5 contdExample #5 contd

What is the limiting reagent?What is the limiting reagent?

How much of the excess (nonlimiting) reagent will be left over from theHow much of the excess (nonlimiting) reagent will be left over from thereaction?reaction?


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Practice Problems and ReviewPractice Problems and Review

Do Practice Problems #1c, 4, 5, and 7 (use answer fromDo Practice Problems #1c, 4, 5, and 7 (use answer from#6) on p.177-178 of your workbook (random HW check)#6) on p.177-178 of your workbook (random HW check)

Finish Vocabulary AssignmentFinish Vocabulary Assignment due day of unit testdue day of unit test

Practice for the Unit Test by doing Review Problems #2-4,Practice for the Unit Test by doing Review Problems #2-4,6-10 and the multiple choice test #1-6 on p.179-181 of6-10 and the multiple choice test #1-6 on p.179-181 of

your workbookyour workbook

Unit Test: November ____Unit Test: November ____

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