http://en.wikipedia.org/wiki/Limiting_reagent

Limiting Reagent- the reagent that is completely consumed in a reaction; the other reactants which are not completely “used up” are said to be “in excess”

A typical problem using the concept of limiting reagent asks for the quantity of reagent A that is required to consume reagent B (or vice versa).

Alert!

Problem-solving that involves a limiting reagent is almost always recognized by the fact that more than one reagent is given in the problem.

For example: A solution containing 15 g of AgCl and a solution containing 25 moles of NaBr were mixed. How much product is theoretically formed?

% Yield -

% yld = (actual yld)/(theor. yld)         X 100%

Theoretical yld is the amount calculated from the stoichiometry of the reaction.  This is the amount of product obtained after reaction “goes to completion”

Late getting home.

The homework is to choose a chemical reaction from chapter 11 and use it to make ratios for dimensional analysis.

You will need to list mole ratios, particles/mole, volume of gas, and molar mass.

Today we finished the labs.  Please finish the calculations and questions for Monday.  Don’t forget we will be having a quiz.  Finish the worksheets.

Lab calculations:

Step 1 :You were given the number of grams of NaOH.  You need to use that to calculate the number of moles of NaOH.  How?  Look at your mole map to see how you can go from mass (grams) to moles.  Once you have moles of NaOH divide that by 1 L to get molarity in moles/liter.

Step 2: Calculate the number of mL of each solution HC2H3O2 and NaOH you used in your experiment.  Then convert this to liters.

Step 3: Ok, you know how many liters of solution for NaOH and the molarity of NaOH.  Guess what?  Your mole map tells you that to go from Liters of Solution to moles you can use molarity.  Set this up as a dimensional analysis starting with the number of liters of NaOH.

Step 4:  Finally, you have moles of NaOH.  Now this is just like the problems we have been doing.  You have calculated moles of NaOH… how do you get to moles of HC2H3O2?  Check out your mole map again to see how to get from moles of known to moles of unknown.

Step 5: Are we there yet?  ALMOST!  Question number 1 says to calculate molarity of HC2H3O2.  In step 4 you found moles of HC2H3O2.  You know how many mL of HC2H3O2 you used in the lab because it is in your data chart.  How can you get molarity from moles and mL?  Think of the units of molarity!  (PS.  This step is also in your handy mole map)

Step 6:  One question left!  Grams of HC2H3O2.  You know moles of HC2H3O2.  How do you get from moles to grams?  Check out that mole map again.  (see how useful it is! don’t lose it, you can use it on your quiz)

THAT’S  ALL!  GOOD LUCK!

Continue Mass-Mass conversion worksheets.

Lab calculations:

Don’t forget to convert from grams of NaOH to moles of NaOH to find the concentration of NaOH used in the titration.  You should have written down 4.193 grams NaOH in 1 liter of solution.  You convert from grams to moles the same way we are doing on the worksheets and in class.  Check mole map if you can’t remember.

HW:  Keep working on the worksheets.  You will have time in class tomorrow to work on the problems and laboratory calculations.  If you did not do the lab today plan on being in the lab Friday.

Although these problems follow the same format as the mole-mole problems, they are more challenging.  You may want to consider really learning how to use the mole map to navigate your problem.

I’ve given you an example… but as I was searching for a new FUN worksheet for you for tomorrow… I found this video.  Ok, it’s a little strange – but the information is great.   http://www.gpb.org/chemistry-physics/chemistry/802         You may want to try this out for review.  We have not covered actual yield so stop watching when they get to actual yield.  THIS WOULD BE GREAT for those of you who are missing class Th and Fr.  They also have one covering mole-mole problems and introduction to mass-mole problems http://www.gpb.org/chemistry-physics/chemistry/801 

Sample Problems

Calculate Molar Mass      Determine number of moles in 30 grams of H2O

Molar Mass of H2O

H= 2*1.01

O=16

Add 2.02+16= 18.02 grams/mole of H2O

Or as an equivalence statement     18.02 grams H2O=1 mole H2O

Use this in a problem:

You would like to make 30 grams of water.  You have H2 and O2.  Write the reaction and solve for the number of grams of H2 you would need.

Given:  30grams H20

Want: grams of H2

Balance Equation:  2H2+O2 =>2H2O

Write an equivalence statement that relates the given and wanted

2 moles H2O = 2 moles H2

Molar Mass Needed:  Given is in grams and want grams so you have to do two molar mass calculations.

Molar Mass of H2 (wanted)

        H=  2*1.01 = 2.02grams H2/1mole H2

Molar Mass of H2O (given) (already calculated above)  is 18 grams/1mole 

Now we have everything we need!  Time to solve.

Starting with given:

30 grams H2O  we need to get rid of grams and get to moles so we use the molar mass of H2O

Now we have the moles of H2O and need to get to mole of what we want.  We use the eq. statement we wrote above.  This gives us moles of H2

Finally we need to get from moles of H2 to grams of H2.  To get from moles to grams we need to use molar mass.  We calculated the molar mass of H2 above. 

Each of these steps needs to go into dimensional analysis so that the units cancel and you end up with the answer!  It’s hard to do dimensional analysis on here but I’ll post it later as a photo if I can.

Good luck and email me if you have questions.

If you are interested in additional practice/review I have created (edited) an online course in stoichiometry.  The login information is below.  I will try to let you know which sections might be helpful in daily posts.  The videos in the next post are probably more helpful but I left this post up in case you are interested.

Chemistry I CSAS

Course Key: Chem1-CSAS

Institution: CSAS

 Access your course at: https://oli.web.cmu.edu/

Today we introduced stoichiometry.  We used a mole map and did mole to mole conversions.

Homework is the mole to mole worksheet problems 1-3.

Lesson topics

What is a mole?

How to use a mole map.

Convert from moles of product to moles of reactant using a balanced chemical equation.

Mole Map

Sample Problems:  Moles to Moles using Mole Ratios and Stoichiometry

Balance each of these equations. 

Use the mole ratio from the balanced equation to find the moles of compound needed

 1.       Mg       +       HCl     gives(I can not do an arrow in this post)    MgCl₂       +       H₂

          How many moles HCl are needed to react with 5 mol Mg?

GIVEN 5 mol Mg

WANT moles HCl

BALANCED EQUATION:  Mg       +       2HCl    yields   MgCl₂       +       H₂

Molar Ratio:   2 moles of HCL = 1 mole Mg

Solve  5 moles Mg | 2 moles HCl                    (It is difficult to show dimensional analysis on here) 

                                      |   1 mole Mg

2.       NaI       +       Cl₂                    NaCl       +       I₂

          Given 6 mol Cl₂, find moles NaCl

Your turn

 3.       KClO₃                   KCl       +       O₂

          How many moles of O₂ are formed from 3.5 moles of KClO₃?

If you want to use the online review course… you may want to view  

module 3 introduction 

module 2 basic tools of stoichiometry pages 1 and 2 

module 5 reaction  stoichiometry pages 1 and 2 (note that we have not done grams calculations yet in class)

This blog will help you study and review.  Keep looking for more information!  I will continue to update this site when I get a chance.  I did my best to choose links with good, appropriate content.  However, I do not control the links I provided and can’t promise they will not change.