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Lesson Sequence: Day 3

Day 3: Making sense of NaCl/CsCl lab data, mass vs number concept.
  1. Class Discussion/Problem Solving

      small eggs jumbo eggs  
    If 1 doz 1 doz =the # of eggs (12)
    What is mass? (60 grams) (180 grams) =the mass of eggs
    What if, 1800 grams 1800 grams =the mass of eggs
    What is the #? 30 doz 10 doz =the # of eggs (12)


    RULE (students derive): If you know the mass/object then,

    1. you can figure out the total mass for a group of those objects if you know the total number, and
    2. you can figure out the total number of objects if you know the mass.

    Molecules and atoms work the same way as eggs only they are smaller and we use moles rather than dozens to group them by number.

    In addition, in order to be able to compare the behaviour of different chemicals to each other, you must know how many molecules of each there are because their sizes will be different.

    This is the importance of the number below each element in the periodic table, i.e.,

      1
    H
    1.008
    The number 1.008 is the mass/group of objects, in this case meaning that one mole of
            hydrogen atoms has a mass of 1.008 grams.

    Instead of having just small and jumbo eggs (two different sizes), however, there are 105 different elements (105 different sized objects!).

  2. Apply concept to NaCl/CsCl LAB -

    Review results -






                = Cs+             = Na+
                = Cl-             = Cl-
    salt concentration:     5.8 gm/100mL 5.8 gm/100mL
    current:     6 mA 18 mA



    Poser: Why were these results different when the same amount of salt was used in each case?
    Students
    (eventually):
    SIZE OF MOLECULES MUST BE DIFFERENT!
    Teacher: In what way?
    Students: Cs must be bigger than Na
    Teacher: How can you prove that?
    Students: Find out how many molecules each solution contains and graph the results in terms of the # of molecules instead of mass.
    Teacher: What do you predict will happen to the graphs of the two salts when you do that?
    Students: They should fall on top of, or close to on top of, each other.
    Teacher: What assumption are you making?
    Students: That a + charge has the same magnitude for different elements.



  3. Students work in their groups to re-graph their data and write a second conclusion based upon their revisions.

  4. Homework - Biology, Ch 3, section 3 (Interactions of Matter), section 4 (Chemical
    Reactions), read and do the review questions at the end of each section.



DAY 1     DAY 2     DAY 4     DAY 5     Teacher Info



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