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«The Young Scientist Program - Teaching Kits Washington University School of Medicine Funding provided by The Leon Lowenstein ...»

The Young Scientist Program - Teaching Kits

http://ysp.wustl.edu

Washington University School of Medicine

Funding provided by The Leon Lowenstein Foundation

Molarity and Serial Dilutions

Teacher Handout

Module Overview

One of the most important skills in science is to calculate dilutions. In the lab, correct

dilutions of solutions are critical to performing accurate science. In medicine, these skills

are necessary to ensuring proper medicine dosage and treatment.

This kit teaches the concepts of molarity and dulutions using Kool-aid and nerds. By using Kool-Aid, watching the color fade in the solution provides a visual way to see what is happening in dilutions. Each individual Nerd candy represents a mole of sugar.

By transferring Nerds from one beaker to another and adjusting the ''volume'' in the beaker, students are simulating dilutions.

Goals

1. Understand the relationship between moles and molarity

2. Be able to calculate molarity

3. Understand and calculate serial dilutions Timing This activity will take about 45 minutes to complete.

Kit Materials (per group of 3 students) 1 packet Kool-Aid 5 50 mL conical tubes Beaker Worksheet 16 “moles” (pieces) of Nerds candy calculator (request if needed) The Young Scientist Program - Teaching Kits http://ysp.wustl.edu Washington University School of Medicine Funding provided by The Leon Lowenstein Foundation Molarity and Serial Dilutions Teacher Guide Lesson Design Tool

Course: Teacher:

Week of: Grade Level:

DIFFERENTIATED INSTRUCTION/CROSS CONTENT

CONTENT AREA:

INTEGRATION

Goals: Big Idea/Concept HOW will you teach the content? (groups, strategies, accommodations, variation, etc.)

1. Understand the relationship between moles -Key vocabulary will be introduced to the class as a and molarity whole

2. Be able to calculate molarity -Everyone will work example dimensional analysis 3.

–  –  –

Essential Question(s)/DOK: Higher Order Questions (scaffolded questions to get to Big Idea)

-Does adding the precise amount of reactant matter when conducting an experiment?

-What could be some consequences of adding the incorrect amount?

-Would you be able to repeat your experiment and its results?

Assessment/Performance Task: How will students Teacher Reflections/Student Work Analysis SHOW what they have learned?

Responses on handout/discussion questions

–  –  –

Background information:

Atomic and Molecular Weight Everything in the world is made up of atoms. Think of atoms as the building blocks for larger structures. There are many different types of atoms (oxygen, carbon, hydrogen, etc.) that combine in specific ways to form molecules. A molecule of water is composed of two hydrogen atoms bound to an oxygen atom. Because an atom is composed of a set number of protons and neutrons, each type of atom has a unique weight. On the periodic table, this is listed as the atom’s atomic weight. When atoms are combined into molecules, that molecule has a weight that is equal to the sum of its individual atoms.

Since an atom is extremely small, scientists are usually concerned with large groups of atoms and molecules. To simplify calculations, the unit of measurement mole was created. One mole is equal to the number of atoms in 12 grams of carbon, which has an atomic weight of 12. The mole was defined so that the mass of one mole of a substance is equal to the substance’s molecular weight in grams. So one mole = 6.02214179(30)×1023 molecules or atoms = molecular weight in grams. Using the mole allows scientists to talk about the number of atoms or molecules in a visible, measureable quantity.

Dilutions

–  –  –

Dimensional Analysis—How to Solve Calculations A tip for doing these calculations is to use something called dimensional analysis. By lining up values with their units, the units can cancel out making understanding the calculation easier. A concentration would be written as 2 grams 1 liter Since 1 liter = 1000 mL

Changing the concentration above to grams/mL:

–  –  –

Remember to save the numerical value next to the unit!! In the end, you must combine all the numerator values and divide by all the denominator values to get the correct answer!

Here’s an example problem to show how to perform these calculations:

I have 20 grams (20 g) of solid with a molecular weight of 2 grams per mole. How many moles do I have?

–  –  –

Here’s another example problem to show dimensional analysis in action:

How many grams NaCl (sodium chloride, table salt!) do you need to make 100 mL of a 2M solution? The molecular weight of NaCl is 58.4. This means there are 58.4 grams of NaCl.

–  –  –

Exercise overview 1: Kool-Aid concentration and serial dilutions This series of problems is designed to teach the concepts of concentration and dilutions. By using Kool-Aid, watching the color fade in the solution provides a visual way to see what is happening in dilutions.





Materials:

1 packet Kool-Aid 5 50 mL conical tubes calculator (optional)

Protocol and Questions:

1. Add the contents of one packet of Kool-Aid to a 50 mL conical tube and label it “Tube 1”. Fill the tube to the 50 mL line with water.

Cap the tube and shake until the Kool-Aid goes into solution.

Why did you fill the tube to the 50 mL line as opposed to adding 50 mL of water?

___________displacement_________________________________________________

–  –  –

How many grams are in the packet? ________ What is the total volume of your solution? __50 mL_____ What is the concentration of your Kool-Aid solution? ___________

–  –  –

3. Take 5mL of the Kool-Aid solution and put it in a new 50mL tube. Label this “Tube 2”.

Did the concentration in “Tube 1” change when you removed the 5mLs? ___no________ Why or why not? __All of the solution is the same concentration____________________

4. Fill “Tube 2” to the 50mL line with deionized water. Compare “Tube 1” and “Tube 2” and record your observations. ______________________________________________

______________________________________________________________________

Calculate the concentration of the Kool-Aid solution in “Tube 2” using the equation M1V1 = M2V2.

Concentration in “Tube 2” ____________

5. Repeat the dilution two more times by transferring 5mL from “Tube 2” to “Tube 3” and filling it to 50mL, then doing the same thing for “Tube 4”. Compare the solutions in all 4 tubes.

What do you observe about them? __________________________________________

______________________________________________________________________

Can you tell which solution has the highest concentration? _yes, it is the darkest______

6. Add 4.5 grams of table salt (NaCl, molecular weight = 58.44 g/mol) to a new conical.

Label this tube “Tube 5”. Fill the tube with water to the 50mL line and shake gently until the salt goes into solution. Visually compare your salt solution to your Kool-Aid solutions.

Based solely on your visual comparison, line up your solutions from the most concentrated to least concentration. Write down your order.

Highest __________________

___________________

___________________

___________________

Lowest __________________

–  –  –

7. Calculate the Kool-Aid concentrations in each tube.

Concentration in “Tube 3” ____________

Concentration in “Tube 4” ____________

Calculate the concentration of the salt solution in “Tube 5”.

Concentration in “Tube 5” ____________

Based on your calculations, line up your solutions from the most to least concentrated.

Highest __________________

___________________

___________________

___________________

Lowest __________________

Did your prediction based solely on observation match the calculated concentrations?

Explain._______________________________________________________________

______________________________________________________________________

Exercise overview 2: Nerds molarity and serial dilutions This series of problems is designed to explain the concepts of moles and molarity. Each individual Nerd candy represents a mole of sugar. By transferring Nerds from one beaker to another and adjusting the “volume” in the beaker, students are simulating dilutions.

Materials:

Beaker Worksheet 16 “moles” (pieces) of Nerds candy calculator (optional)

Protocol and Questions:

1. Place 16 moles of sugar (1 Nerd = 1mole) onto beaker 1 on your worksheet. Imagine adding 1 Liter of water to beaker 1. Using your pencil, shade beaker 1 to represent this volume.

How many moles are in beaker 1? ___16______

–  –  –

2. Transfer 1/2 of your solution to beaker 2 by moving moles and using an eraser to decrease the volume in beaker 1. In beaker 2, use your pencil to shade the volume transferred from beaker 1.

How many moles of nerds are left in beaker 1? __8__ What volume is left in beaker 1? _500 mL___ What is the molarity (moles / liter) in beaker 1? __16____ Did the molarity in beaker 1 change after you transferred 1/2 of your solution to beaker 2?

__no___ Explain your answer! _All of the solution is the same concentration____________

______________________________________________________________________

What is the molarity (moles / liter) of the nerd solution in beaker 2? ____16_________

3. Shade beaker 2 to represent filling the beaker to 1 Liter with water. Re-calculate the molarity of the nerd solution in beaker 2 and record your answer under beaker 2.

4. Transfer 1/2 of your nerd solution in beaker 2 to beaker 3. Using your pencil, shade beaker 3 to represent filling the beaker to 1 Liter with water. Calculate the Molarity of the nerd solution in beaker 3, and record your answer under beaker 3.

5. Transfer 1/2 of your nerd solution in beaker 3 to beaker 4 and use your pencil to shade beaker 4 accordingly.

Oooops! You forgot to put away beaker 4 before going home last night. Evaporation has caused the volume in beaker 4 to go down to 125 mL! Use your eraser to represent the new volume in beaker 4.

Did you lose any nerd moles to evaporation? __no____ Explain your answer! __The_water evaporates, but the nerds do not since their vapor pressure is too low._____________________________________________

How many moles are in beaker 4? ____2________ Calculate the Molarity of the nerd solution in beaker 4, and record your answer under



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