Apologia Chemistry: Moles and Percent Composition

I started class by defining a mole.  The analogy I used was train cars.  A train car of marshmallows has the same volume as a train car of nails or bricks. Another analogy for moles is a dozen.  I explain that two train cars may have the same number of nails marshmallows in each.  The two train cars have the same volume and the same number: however, their masses are different.  That is the concept of a mole.  I explained that Fe, iron has an atomic weight of 55.8.  One mole of iron has 55.8 grams.  I held up a box of nails and asked if there might be one mole of iron in the box.  There may be depending on the number of nails in the box.  I have 30 g samples of cupric chloride bottles.  There is not a mole in the bottle.  This is my basic introductions.  On Thursday, we will use their masses (5-10 g) and calculate the number of moles.


Today and Thursday we are doing labs.  First, the kids weighed moles: iron nails, copper shot, aluminum foil, mossy zinc, baking soda, and salt.  They created simple data tables: name, formula, mass, and molar mass.  It helps students to understand what a mole is when they weigh moles.  We are doing the calculations on Thursday.  I plan to both calculate the theoretical and expermental  percent of water in each of the four hydrates.

Next, the kids collected the data for Percent Composition Quick Lab.  The hydrates we used are copper II chloride dihydrate, copper II sulfate, iron III chloride hexahydrate, and zinc sulfate monohydrate.  It’s important to weigh the test tube before heating.  Sometimes the chemical cannot be removed after it’s heated.  We used sterno instead of tea candles to heat the hydrates.  Last week we heated copper II sulfate pentahydrate hydrate to demonstrate decomposition chemical reactions with tea candles I successfully.  Sterno worked well; it’s much more stable than an alcohol burner.  Remind the kids that hot glass looks the same as cold glass.   Remind the kids to angle the test tube away from people as they gently heat the hydrates.   Allow the test tube to cool in the test tube rack.  The hot test tube can melt the plastic digital scale.  Remind the kids to use weigh paper or a weigh boat: chemicals should not be weighed directly on a scale.  Remember the kids are teens; these instruction may not be obvious to kids.

Hydrates  have water loosely bound ionic compounds.  Hydrates can act as dessicants.  The salt, say copper II chloride is anhydrous.  The hydrate in this example is copper II chloride dihydrate.  The anhydrous salt can absorb water from its surroundings.

The kids are learning about percent composition of formulas by heating and removing water from several hydrates.    We do the math together. BTW, the topic of hydrates are not included in Apologia Chemistry textbooks.  I think hydrates are useful to understand percent composition.