Masses of Chemicals

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Objective

SWBAT calculate molar masses of substances based on their chemical formula using a current periodic table.

Big Idea

Every chemical has a unique molar mass based on its chemical composition.

Background

Today is essentially a return to unit 2, using the periodic table to calculate the molecular and molar masses of compounds.  We will open the period with a quick hands on so students can understand why the periodic table has to be used.

For the mole jars, the following are what we use:

  • Copper turnings  63.55g
  • Carbon powder  12.01g
  • Lead shot  207.2g
  • Aluminum shot 26.98g
  • Lump sulfur 32.00g
  • Silicon 28.09g
  • Iron filings 55.85g
  • Mossy zinc 65.38g
  • Sheet tin 118.71g

All are relatively inexpensive, keep well, and are safe if students should happen to open the jars.

This lesson runs for 42 minutes due to our monthly morning resource hour schedule.

This lesson is the prerequisite for a study of HS-PS1-7, using mathematical (SP5) representations to support the claim that atoms, and therefore mass, is conserved during a chemical reaction.  The use of mass to count atoms directly aligns with the Scale, Proportion and Quantity Cross Cutting Concept at the middle school level (Proportional relationships among different types of quantities provide information about the magnitude of properties) and high school level (Some systems can only be studied indirectly as they are too small to observe directly).

Opener

10 minutes

When students come in, I ask a representative from each table to come get two jars of chemicals from the front of the room.  I tell students to not open them, but to observe the jars for similarities and differences.

Depending on which jars have, groups may say:

  • All metals
  • Both are powders
  • Both are solids

I then have them switch with another table, so they can observe two additional jars.  New responses include:

  • The textures are different
  • The colors are different
  • They are different amounts

On the last one, I ask "What do you mean, they are different amounts?"  Students will say:

  • One is heavier than the other
  • One jar is more full than the other

I acknowledge that both of those are true.  Then I tell students that all of the jars have one mole of the element listed.  So while they are right that some are heavier, or appear to take up more space, that they all have the same number of atoms.

I then ask "What does this tell us about different atoms?" and students respond:

  • They are different sizes
  • They are different weights

I tell them that both are true, and that we learned that first semester when we learned the periodic table.  I ask students to get out their periodic table, or to get one off the front table while I pass out the paper for the day.

Masses of Chemicals

10 minutes

Once students all have their Masses of Chemicals paper, we begin by looking at the first steps.  I ask students to read the opening paragraph while I put a copy of the paper on the document camera, and project it so students can see it.

After everyone has read through, I ask:

  • "How many types of masses are we working with?"
  • "Two"
  • "What are they?"
  • "Formula and molar"
  • "What's the difference?"
  • "The unit, because formula only does a single atom or molecule."

After this exchange, we start to look at the first example, potassium permanganate.  I ask students to look up and write down the mass of each exactly as written on our periodic table.

Next, we break down the chemical formula, identifying 1 potassium, 1 manganese and four oxygen atoms.

Next we multiply the masses by the number of atoms, and then add all those together.

Once students have the answer of 158.032, they record the number for both the formula and molar mass.  I ask them what unit belongs on the formula mass, and they give us amu to label it.  I remind them that in the word "Formula" they can find the unit.  Same for "molar" mass to remember that they are representing the number of grams in one mole (g/mol).

I then give them time to practice on the calcium nitrate example.  Some students still struggle with the chemical formulas, not separating the nitrogen and oxygen, others not distributing the 2 to both parts of the anion.  Overall, students do well in translating from the periodic table to this process. 

Students express that it is tedious to have to write and rewrite the numerical answer with two different units.  I agree, and point out that once they are masters at it, they won't have to.

Independent Practice

22 minutes

The remainder of the period, I allow students to work on the 16 practice problems.  I require them to complete 1-12.  While students are working, I am checking in with them and checking their work to make sure they are:

  • Separating the formula into the elements correctly
  • Counting the atoms correctly, especially where polyatomic ions are concerned
  • Putting correct units for each the formula and molar mass.

When students get to 13-16, they freak out at first, not realizing that the name is just the common name for the household chemicals.  Since we didn't teach nomenclature this year, I didn't put the names of all the chemicals, just those that are common.  I didn't anticipate students freaking out about not finding "Sugar" on the periodic table.  When I point out that the process is the same, that they can ignore the name and work with the formula, they do just fine.