Molarity

Continuing with the Solution theme, today's post is devoted to the Solution topic of Molarity.

Objectives:

1.  Describe how to prepare a solution of a certain molarity.

2.  Calculate molarity (M), moles of solute or liters of solution given the other two variables.

3.  Calculate the amount of concentrated solution to mix a given amount of dilute solution.

4.  Solve stoichiometry problems involving solutions of known molar concentrations.

5. Identify the limiting reactant (or reagent) in a chemical reaction and predict the amount of precipitate formed.

The solution concentration molarity is used a great deal by the chemist.  That is because by measuring the volume of the solution, you can use the molarity to calculate exactly how  much solute is contained in the sample. 

Here are two videos about mixing solutions using a volumetric flask.  This first video addresses the calculation quickly.

This second video talks a little more about technique:

Notice how in both videos they are wearing aprons and goggles.  One chemist is wearing gloves.  I've never used them b/c of the chance of the glove slipping so the glassware is dropped, and the possibility of a chemical getting inside the glove and directly on your skin.

Both videos show the use of plastic weighing dishes.  They're a cost of about $20 to $30 for 500 -- they're not typically available in a high school laboratory.  I just used pieces of scrap paper --- 8" x 11", cut in half -- I would fold the paper in half to aid in keeping the chemical on the paper, and for pouring the chemical into a volumetric flask.  Once used, the paper was simply thrown away.

Here is a 12 minute video solving mathematical problems about Molarity.  There are 4 problems, so you might not need to view all of them

Here's a practice problem worksheet, and it's got the answers at the bottom!  Bonus!

http://ptec107.wikispaces.com/file/view/S03_molarity+ws.pdf

It's very typical to order a stronger solution for your stockroom and dilute the solution according to what is needed.  Here is a brief video about dilutions:

Not all high school labs have too many pipets to use -- just do the best you can.  Because the high school lab is not typically performing research, the accuracy may not be as good as a research lab.

This is a 7 minute video (by my favorite, Mr. Post) about solving dilution problems:

Here is worksheet of Dilutions problems:
http://www.nclark.net/DilutionsWorksheet.pdf

An important application of molarity is to the math of chemical reactions (stoichiometry).  Here are a few worksheets:

http://vanmaarseveen.weebly.com/uploads/6/2/6/5/6265786/15-6solnstoichiometry.pdf

http://misterguch.brinkster.net/PRA048.pdf

This worksheet is several pages long, but it has several pages of examples with full solutions, and examples with  answers.   Perhaps you want to give the online link to your students as a supplement:

http://www.eiu.edu/eiuchem/GenChem/downloads/tutorial4.pdf

One lab that I put together in one school was to have the students calculate and mix solutions for a simple  Na2CO3 + CaCl2 reaction.  This was only because I had enough 100 mL volumetric flasks available for the students to mix their own solutions.

Perhaps you could have the class calculate and aid in mixing solutions during a class demonstration, then the students use the solutions in a 2nd laboratory.

The CaCO3 precipitate was very fine and some passed through the filter paper.  I would try larger concentrations to see if the results were improved. 

I found several examples of labs using this reaction, and here's similar lab using strontium nitrate instead of  calcium chloride:
http://www.pleasanton.k12.ca.us/fhsweb/morris/chemlab/Stoich/Stoich%20lab.pdf

This is 10 minute video discussing the concept of limiting reactant (or reagent):

Here's a page of practice problems about limiting reactants with answers:

http://teachers.sduhsd.net/jnewman/Gen%20Chem%20Worksheets/Practice%20Limiting%20Reagent%20and%20Percent%20Yield%20Problems.pdf

This is a longer page on limiting reactants which also has answers:

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

*I'd love to hear from you!  Your feedback would really help me to focus on your needs, so write to me!   Simply click on the "Contact" tab.  

Check out my lab book "Chemistry on a Budget" at amazon.com:
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

Each lab is presented with two possible report formats -- both with the same procedure -- one with 10 questions to be answered as a conclusion, the other with a full laboratory report required.  This was to give the teacher the option of what type of report is desired! 

Have a good week!