Slide 1
Count particles by the mole
A mole is an amount containing exactly 6.02214076 times ten to the twenty-third specified entities. The entities might be atoms, molecules or ions.
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The word specified matters: one mole of oxygen atoms and one mole of oxygen molecules do not contain the same number of oxygen atoms. Naming what is counted prevents a factor-of-two error.
Slide 2
Mass makes counting practical
Molar mass tells how much mass corresponds to one mole of a substance. A balance can then connect a macroscopic sample with particle amount.
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Dividing sample mass by molar mass gives amount in moles. For an illustrative substance with molar mass 20 grams per mole, a pure 10-gram sample contains half a mole.
Slide 3
A formula gives internal ratios
A chemical formula states the relative numbers of atoms or ions in a substance. These ratios apply to mole amounts as well as individual units.
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One mole of water molecules contains two moles of hydrogen atoms and one mole of oxygen atoms. It does not contain separate hydrogen and oxygen gases; the atoms are bound within water molecules.
Slide 4
Reactions compare amounts
Balanced reaction equations relate amounts of reactants and products. The coefficients count particles or moles, rather than equal masses.
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If a reaction consumes two units of one reactant for every unit of another, equal masses need not be the correct mixture. Molar masses are required before mass measurements can be compared with that ratio.
Slide 5
Real samples need characterization
Purity, isotopic composition and measurement uncertainty affect a mass-to-moles calculation. A stated sample mass does not by itself guarantee the desired particle count.
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Water absorbed by a powder adds mass without adding the intended compound. Analytical work therefore distinguishes total sample mass from the mass fraction of the substance whose amount is being calculated.
Sources
- Definitions of SI Base Units
National Institute of Standards and Technology
- Why Does Water Dissolve Sugar?
American Chemical Society