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?