Misconceptions
The Mole and the Avogadro constant
A mole of one substance will have the same mass and volume as a mole of a different substance.
Mass and the Mole
Oxygen has a molar mass of 16 g/mol.
Empirical and Molecular Formulas
The molecular formula is determined using a
different strategy than that used to determine
the empirical formula.
What Is Stoichiometry?
You can calculate the required masses of the
reactants for a chemical reaction and predict
the products directly from the balanced
chemical equation; there is no need to involve
moles.
Stoichiometric Applications
As long as the reactants for a reaction are
used in stoichiometric amounts, you will get
exactly the same products for the reaction in exactly the same amounts each time.
All sections
Units do not matter as long as the answer to a
problem is correct.
Students have learned that two 1 m lines will look the same length, and two 1 kg objects will feel equally heavy. But two 1 mol samples may look or feel like very different amounts. Explain that a mole is always the same number of particles, but because the particles may be atoms, molecules, ions, or formula units, the particles are of very diff erent sizes and masses. Ask questions such as the following: “A mercury atom has a mass almost 200 times that of a hydrogen atom. Which will be heavier, a mole of mercury or a mole of hydrogen? A hydrogen nucleus is just one proton, while a uranium-238 atom has 92 protons and 146 neutrons. The diameter of the uranium atom is about eight times that of the hydrogen atom. Which will take up more space, a mole of hydrogen or a mole of uranium-238?”
or another way to overcome this misconception is to show 1 mole of different substances like below.
Explain that molar mass is always given for the element in its standard state (phase at standard temperature and pressure). In its standard state oxygen is a gas, O2. O2 is a diatomic molecule, so the molar mass of oxygen is the mass of one mole of O2: MO2 = 2(16.00 g/mol) = 32.00 g/mol Students can use HOFBrINCl to remember the seven diatomic elements
Teach the empirical formula first, and then the molecular formula, to reinforce the
procedures.
A balanced chemical equation shows the proportional relationship between the numbers of particles in the reactants and products of the reaction. The terms of the ratio represent number of individual atoms, molecules, or formula units; simply multiplying each term by NA gives the ratio of reactants and products in moles. With this ratio, the molar masses can be used to determine the mass of the reactants and products in grams.
Provide simple concrete examples. For example, the ratio of staples to test papers is
1:1. The mass of the test paper is 5 g. Do you need 5 g of staples for each test paper?
No. The mass of one staple is about 0.05 g, so 5 g of staples would be about 100
staples for each test paper. The 1:1 ratio means that you need one staple for each
test paper.
Have students brainstorm factors that could cause the amount of cookies produced
from one batch of a recipe to be less than expected (spilled dough, eaten dough,
and so on) or could cause the product to differ from what was expected (dropped
cookies will break, a too hot or too cool oven will lead to burnt cookies or cookies
that didn’t rise, and so on).
Without units, the answer cannot be correct. To reinforce this, always show units
when completing work on the board. Assign marks for having complete and correct
units in all solutions.