2/22 Chemistry recording - Limiting Reagents

https://youtu.be/v9HtPoJPMM0

2/22 Chemistry Class and homework due 3/1

HI folks,

Today we took a look at our last step in the world of stoichiometry which is limited reagents.  We also did a fairly fast but interesting lab where we found how much carbon went into and came out of a reaction with sulfuric acid and baking soda.

Here are a couple of videos to help you if you need it.  One is the video we saw in class today and the other is the same fellow solving a limiting reagent problem that involves masses of compounds.

Limiting Reagents Video from Class https://www.youtube.com/watch?v=nZOVR8EMwRU

Same fellow solving a problem with masses https://www.youtube.com/watch?v=N0dTXcoHI-I

For homework this week there are 3 different short assignments.

1. For homework this week do the following questions based on today's lab.  Since both groups got .2 g of CO2 feel free to use that as your mass generated.  I've included  a photo of the answers as well.

2. Answer the limiting reagent questions in the email 

3. Answer the limiting reagent questions in the first worksheet in the enclosed .pdf.  Please feel free to use the other problems as practice.  Answers for this are also included.

Questions based on the lab

1.  Find the percentage composition of carbon in CO2.

2.  Look at your data and, using the percent composition of carbon in CO2 (the answer you found for #1) find the amount of carbon you had in your final mass of CO2 generated.  In other words, multiply your mass of CO2 generated by the percentage of C in CO2.

3.  Find the percentage composition of carbon in Sodium Bicarbonate.

4.  Using the answer you found in #3, find the amount of carbon you started with.  In other words, take your mass of Sodium Bicarbonate and multiply it by the percentage you found in #3.

5.  The answer in #2 is the mass of carbon you ended up with.  The answer to #4 was the amount of carbon you started with and (thanks to the law of conservation of mass) you should have ended up with.   Chances are you lost some carbon along the way and so, with that in mind, you need to find your percentage error.  To do this take the mass you ended up with (answer #2) minus the mass you started with (answer #4) and divide that number by the what you started with (again answer #4).  Multiply that by 100 and that's is your percentage error.

In other words ( (Mass you got - mass you should have gotten)/Mass you should have gotten) x 100 = percentage error.  You want a small number here.  

6.  Why should answer #2 and answer #4 be the same?

Limiting Reagent Questions 

1.  4NH3 + 5O2 —> 4 NO + 6H2O

a.  If you start with 6 moles of NH3 and 9 moles of O2, which is the limiting reagent?

b.  Which is the excess reagent and how much many moles would you have left?

c.  How many moles of NO do you make?

2.  2 Al + 3 Cl2 —> 2AlCl2

a.  If you start with 4.5 moles of Al and 3.2 moles of Cl2 which one is the limiting reagent?

b.  How many moles of excess reagent do you have left?

c.  How many moles of AlCl2 do you get?

3.  2Fe  + 3S —>  Fe2S3

a.  If you start with 2.6 moles of Fe and 1.6 moles of of S which one is the limiting reagent?

b.  How many moles of excess reagent do you have?

c.  How many grams of excess reagent do you have?

d.  How many moles of Fe2S3 do you get?

e.  How many grams of Fe2S3 do you get?

Help for the chem homework this week

Hi folks,

I'm including the steps to these empirical formula problems.  They are the same thing that you should have in your notes but  just in case…. :)

Also, I'm going to indicate which problems each set of steps goes with for the homework worksheet.  Hopefully this will help make these a bit smoother to tackle. 

A. Finding empirical masses if you know masses of elements

1. Divide masses of the elements given in the problem by their molar mass
2. Divide all answers by smallest answer found in #1.
3. Find smallest whole # for all

B. Empirical formulas with percentages  Problem 1,2

1. Assume you have 100 g of the compound so that the percentages can be the same as masses.
2. Divide each percentage by it's molar mass
3. Divide all answers by smallest answer found in #2.
4. Find smallest whole #'s for all atoms in the molecule.

C. Empirical formulas with mass of molecule  Problems 3, 4, 5 (remember percentages can be used like  masses)

1. Divide mass of each element given by the total mass of molecule to find percentage of element in the molecule.
2. Divide those masses by the molar mass of each element.
3. Divide all by smallest answer found in #2
4. Find smallest whole #'s for all

D. Going from Empirical to Molecular from molar mass of molecule  Problems 6, 7, 8

1. Find the molar mass of the empirical formula
2. Divide molar mass given by empirical formula's molar mass.
3. Multiply each atom in the empirical formula)by answer to find Molecular formula

E. Going from empirical formula to molecular formula from masses of elements in compound  Problems 9, 10

1. Find empirical formula of compound (See either A or B)
2. Find Molar Mass of the empirical formula.
3. Divide molar mass given in problem by empirical formula molar mass
4. Multiply the atoms in the empirical formula by answer in step 3 to find molecular formula.

2/15 Chemistry Class and homework due 2/22

Hi folks,

This week we took the time learn how to work with empirical formulas in every way we possibly can.

For homework this week do the questions 1 - 10 on the first page of the worksheet at this link.  Answers are included but he does use a different system than we use in class to solve them.

There are more problems on the sheet if you would like to practice them but they are not assigned. 

If you'd like to read a bit about empirical and molecular formulas I'd highly recommend this page.  It does a quick, nice job of explaining what empirical and molecular formulas are and it's hilarious!  There are also some worked examples there to take a look at.

Recording of 2/15 Chemistry Class

https://youtu.be/Klca9MMez6I

Unfortunately, the last bit of the lecture was not recorded since I turned the camera off when we started that and I didn't turn it on again.

1/8 Chemistry Recording - Percent composition and Empirical formulas

https://youtu.be/OcG2RpMVLeA

1/8 Chemistry update and homework due 1/15

Hi folks,

Today we we covered a couple of quick but important topics in chemistry.  Percent composition and empirical formulas.  Next week we will dive a bit deeper into empirical formulas as well

For homework this week please do the enclosed .pdf.  The answers are enclosed so see if you can really master these before next week.

2/1 Chemistry Class update and homework due 2/8 Take Home Test!!!

Hi folks,

We spent a bit more time reviewing stoichiometry today.  It's tricky stuff but the more you work with it the easier it gets.  We also did a lab today where we put much of the material we've been learning this year to the test in order to measure the width of a molecule using nothing but soap, water and pepper.

For homework this week, do the following take home quiz.  This will be worth the same amount as our classroom quiz so please spend quite a bit of time with it to get a good score.

Good Luck!

2/1 Chemistry Recording - Stoichiometry review

https://youtu.be/ZIQQKX8Hgto