Blog Archives

Percent error

12/29/2014

Happy New Year! I hope you enjoyed the holiday and got some good rest! I decided to post a little earlier, just in case you were researching before going back to school next week.

You might have received gift cards or cash as a gift for the holiday. Remember that you can buy my lab book "Chemistry on a Budget" for $20.54 at amazon.com or $23 at lulu.com. It will take a few weeks to get to you, so order now!

http://www.amazon.com/Chemistry-Budget-Marjorie-R-Heesemann/dp/0578129159/ref=sr_1_1?s=books&ie=UTF8&qid=1389410170&sr=1-1&keywords=chemistry+on+a+budget

http://www.lulu.com/shop/marjorie-r-heesemann/chemistry-on-a-budget/paperback/product-21217600.html

This book contains 13 labs using consumable materials purchased from local stores. There are two versions of each lab, one with a ten-question conclusion and one with directions for a full lab report. This way the teacher has the option! Each lab is two pages to allow for one two-sided handout.

Another way to assess the quality of your lab results is to calculate the Percent Error of the result. The formula for Percent Error is:

Percent error = measured value – accepted value x 100% accepted value

Reminder, the % button is not used on the calculator for this problem.

I did not include the absolute value brackets – some sources don’t include it – but I was taught to use it so it’s sort of hard to ignore it.

Here is a brief video (7 minutes) about this topic:

This narrator points out the use of absolute value brackets, and breaks down the formula to aid comprehension.

Here are a few worksheets to practice percent error calculations:

This page has mathematical questions with answers and also asks a few questions about accuracy and precision.

http://www.sfponline.org/Uploads/382/acc_pres_percent_error_half-sheet.pdf

For this worksheet, notice the directions at the top of the page require that work and units be shown.

http://www.polk.k12.ga.us/userfiles/646/Classes/4526/Percent%20Error%20Practice%202.pdf

This worksheet saves paper because it has the worksheet on the two halves of the page, and it has answers. (I hope you can find the paper cutter in your department!)

http://mrnorton.com/assignment_calendar/PercentErrorPractice.pdf

Check out the Topic List to help you to find past Blog entries. I hope it helps!

Enjoy the rest of the holiday! I plan to discuss Midterm Examinations next week – I did post about Final Examinations on 06/08/2014. It may help!

Have a good New Year celebration!

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limiting reactant and percent yield

12/18/2014

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My last post on 12/10/2014 was about Stoichiometry. A typical problem using Stoichiometry is to determine the Limiting Reactant in a situation.

A Limiting Reactant (or Reagent) is the reactant that produces the least amount of product. A typical mathematical problem has values for two reactants – it is the chemist’s responsibility to calculate both amounts of product to determine the Limiting Reactant.

Upon producing a substance in lab, it is typical for a student to be asked to evaluate the quality of the lab result. One typical way of performing this is calculating the Percent Yield. This is a way to evaluate the quality of the experiment by examining the amount produced.

A formula for Percent Yield is:

% yield = Experimental Yield / Theoretical Yield x 100 %

Reminder, the % button is not used on the calculator for this problem.

Using Stoichiometry, a value for Theoretical Yield can be determined. Lab results produce an Experimental Yield. This calculation provides useful information as to the quality of lab results.

If percent yield is less than 100%, reflection/discussion would focus on how the mass was lost. Typical answers could focus on the rinsing of the product, the quality of filtration, and an incomplete reaction.

If percent yield is greater than 100%, reflection/discussion would focus on how the mass was gained. Not meaning to be repetitive, typical answers could also focus on the rinsing of the product, the quality of filtration, and an incomplete reaction.

This following video explains Limiting Reagent and Percent Yield and solves a few problems:

I like how he focuses on moles to figure out the Limiting Reactant.

Here are a few worksheets focusing on Limiting Reagent and Percent Yield:

This worksheet has several examples (and answers) with enough space between problems where it could be projected for use as a classroom lesson. Notice that you’ll have to balance the reactions!

http://www2.palomar.edu/stem/WorkshopMaterials/Chem115/HO%205%20Limreact%20practice.pdf

This is several pages of problems and answers with solutions!

http://msmcraesscience.weebly.com/uploads/5/1/4/4/5144948/limiting_reagent_worksheet__14.pdf

This page contains several problems with space for solving the problems:

http://www.hemethigh.com/apps/download/1Q3LNrmRnm3KhN1UDFQ4vRAwjOnmPxUVUi6W46dA3VEeLJiF.pdf/Limiting%20Reactant%20Worksheet.pdf

Here are several limiting reagent / percent yield problems with answers:

http://theoakwoodschool.org/blog/kthompson/files/2011/08/Limiting-Reagent-Worksheets.pdf

Check out the Topic List to help you to find past Blog entries. It can be a great help!

Most of you are preparing for a holiday break. If you’re a new teacher, congratulations! You’ve gotten this far! And safe travelling for your holiday!

*During the holidays, buy my lab book "Chemistry on a Budget" -- it is available for $20.54 at amazon.com or $23 at lulu.com. It will take a few weeks to get to you, so order now!

http://www.amazon.com/Chemistry-Budget-Marjorie-R-Heesemann/dp/0578129159/ref=sr_1_1?s=books&ie=UTF8&qid=1389410170&sr=1-1&keywords=chemistry+on+a+budget

http://www.lulu.com/shop/marjorie-r-heesemann/chemistry-on-a-budget/paperback/product-21217600.html

This book contains 13 labs using consumable materials purchased from local stores. There are two versions of each lab, one with a ten-question conclusion and one with directions for a full lab report. This way the teacher has the option! Each lab is two pages to allow for one two-sided handout.

Have a good holiday! I’m taking a vacation too, but I plan to post again on January 2nd, 2015.

Happy New Year!

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stoichiometry

12/10/2014

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During the end of November, 2014, I reviewed five types of chemical reactions and predicting the products: Synthesis on 11/14/2014; Decomposition on 11/17/2014; Single Replacement on 11/21/2014; Double Replacement on 11/23/2014; and, Combustion of a Hydrocarbon on 11/25/2014.

With this information and a balanced chemical reaction, you can calculate the theoretical (ideal) yield of a chemical reaction, perform the lab and compare it to an actual yield of product collected.

A useful explanation of “stoichiometry” is:
“Stoichiometry is a section of chemistry that involves using relationships between reactants and/or products in a chemical reaction to determine desired quantitative data. In Greek, stoikhein means element and metron means measure, so stoichiometry literally translated means the measure of elements. (http://chemwiki.ucdavis.edu/Analytical_Chemistry/Chemical_Reactions/Stoichiometry_and_Balancing_Reactions )

Stoichiometry can be approached a few different ways – I find that using Dimensional+ Analysis or The Factor-Label Method (discussed on 09/02/2014) is a very clear way for students to work on these problems.

Here is a 16 minute video about Stoichiometry problems. The beginning discussion of conversion factors is for the use of Dimensional Analysis in solving these problems:

I approach these problems with the same math, but focus on the problem left-to-right just because it’s the way we read English. I like to write the mole ratio under each part of the chemical reaction. For example,

                      2 Mg + 1 O2 à 2 MgO

                           2      :     1      :     2

I also point out that all of the conversion factors combine to form one big fraction. For example,

3.00 g Mg      1 mol Mg        1 mol O2      32 g O2
-------------------------------------------------------------------   = 1.98 g O2
                       24.3 g Mg       2 mol Mg     1 mol O2

Here are some worksheets using the skills of balancing equations and stoichiometry:

This worksheet only has two problems, but it could be projected for classroom lecture use:

http://woodridge.k12.oh.us/ourpages/users/dweaver/Chemistry/PracticeWorksheets/ReactionWorksheets/StoichiometryWorksheet1.pdf

The page contains an overview of the skills required for stoichiometry and answers:

http://academic.evergreen.edu/curricular/matterandmotion/chem_phys%5Cpractice_problems.htm

This worksheet with answers focuses on gas stoichiometry problems:

http://wolgemuthe.psd401.net/chemistry/09%20-%20pvt/documents/Gas_Stoichiometry%20Practice%201.pdf

This post contains several pages and answers related to stoichiometry:

http://chemislife.weebly.com/uploads/9/6/2/7/9627863/stoichiometry_worksheet.pdf

This posting is a good resource of stoichiometry questions and answers:

http://plaza.ufl.edu/ctoyota/worksheet%2013cgt.pdf

Check out the Topic List to help you to find past Blog entries. I hope it helps!

The holidays approach, so buy my lab book "Chemistry on a Budget" (or ask for it as a gift) -- it is available for $20 at amazon.com or $23 at lulu.com. It will take a few weeks to get to you, so order now!

http://www.amazon.com/Chemistry-Budget-Marjorie-R-Heesemann/dp/0578129159/ref=sr_1_1?s=books&ie=UTF8&qid=1389410170&sr=1-1&keywords=chemistry+on+a+budget

http://www.lulu.com/shop/marjorie-r-heesemann/chemistry-on-a-budget/paperback/product-21217600.html

“Chemistry on a Budget” includes the lab A Chemical Reaction that focuses on the stoichiometry of a simple reaction that provides very good results!

This book contains 13 labs using consumable materials purchased from local stores. There are two versions of each lab, one with a ten-question conclusion and one with directions for a full lab report. This way the teacher has the option! Each lab is two pages to allow for one two-sided handout.

Have a great weekend!

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Current automobile airbag concerns

12/4/2014

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Because Takata airbags are in the news lately, I’m going to talk about the chemistry of airbags. This issue relates a lot to kinetics because it is such a rapid reaction, but there is a chemical reaction to focus on as well.

Here is a 9 minute video sequence talking about the basics of airbags:

The phrase “a bomb in your car” describes the concern of the airbag accidentally igniting. And they mention the potential safety hazard of metal shards injuring the driver which is an issue with the Takata airbags.

Here is a CBS news article citing ammonium nitrate, NH4NO3, as the propellant in the airbag:
http://www.cbsnews.com/news/takata-public-best-served-by-regional-airbag-recall/

From http://www.thechemicalblog.co.uk/how-do-airbags-work/ the basic chemical reactions of the typical airbag are presented:

"Under room temperature, sodium azide is a stable compound. However, when heated by an impulse it disintegrates to produce sodium (Na) and nitrogen (N2). It only takes about 50-100 grams sodium azide to produce enough nitrogen gas to fill a normal air bag for the driver (it takes a little more for the passenger).

This, however, is not the only chemistry involved. Unfortunately, sodium will quickly react with water to form sodium hydroxide (NaOH), which can be toxic if inhaled. To minimize this danger, manufacturers add other chemicals, including potassium nitrate (KNO3) and silicon dioxide (SiO2), which can react with sodium, to eventually produce a safe and harmless compound, silicate glass (Na2O3Si).

(1) 2 NaN3 → 3 N2 + 2 Na

(2) 10 Na + 2 KNO3 → N2 + K2O + 5 Na2O

(3) K2O + 2 SiO2 + Na2O → K2O3Si + Na2O3Si (silicate glass)

Most car-manufactures prefer KNO3 and not NaNO3, as it’s less hygroscopic. To ensure the air bag is fully operational even in the presence of moisture, it’s essential to choose non-hygroscopic components, otherwise the reaction may fail."--

Takata is the only company producing airbags using ammonium nitrate. Here is a document discussing safety issues with ammonium nitrate:
http://www.epa.gov/oem/docs/chem/ammonitr.pdf
--
Ammonium nitrate reacts with metal hydroxides, releasing ammonia and forming alkali metal nitrate:

NH4NO3 + MOH → NH3 + H2O + MNO3 (M = Na, K)

Ammonium nitrate gives ammonium chloride and nitric acid upon reaction with hydrochloric acid:

NH4NO3 + HCl → NH4Cl + HNO3

Ammonium nitrate leaves no residue when heated:

NH4NO3 → N2O + 2H2O

Here is a video showing an ammonium nitrate explosion:

The Takata airbag concern is the explosion of the airbag, typically in humid climates, and, schrapnel is being produced by the airbag that is injuring drivers.

*This might be a timely research project for your students. Also, students are always looking for extra credit opportunities.

Check out this website's Topic List to help you to find past Blog entries. It can help in finding other topics you need!

The holidays approach, so buy my lab book "Chemistry on a Budget" (or ask for it as a gift) -- it is available for $20 at amazon.com or $23 at lulu.com. It will take a few weeks to get to you, so order now!

http://www.amazon.com/Chemistry-Budget-Marjorie-R-Heesemann/dp/0578129159/ref=sr_1_1?s=books&ie=UTF8&qid=1389410170&sr=1-1&keywords=chemistry+on+a+budget

http://www.lulu.com/shop/marjorie-r-heesemann/chemistry-on-a-budget/paperback/product-21217600.html

This book contains 13 labs using consumable materials purchased from local stores. There are two versions of each lab, one with a ten-question conclusion and one with directions for a full lab report. This way the teacher has the option! Each lab is two pages to allow for one two-sided handout.

Have a good weekend!

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Percent error

limiting reactant and percent yield

stoichiometry

Current automobile airbag concerns

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