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6.1: The Mole and Avogadro’s Number

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    Learning Objectives
    • Describe the unit mole.
    • Relate the mole quantity of substance to its mass.

    So far, we have been talking about chemical substances in terms of individual atoms and molecules. Yet we do not typically deal with substances an atom or a molecule at a time; we work with millions, billions, and trillions of atoms and molecules at a time. We need a way to deal with macroscopic, rather than microscopic, amounts of matter. We need a unit of amount that relates quantities of substances on a scale that we can interact with.

    Chemistry uses a unit called mole. A mole (mol) is a number of items equal to the number of atoms in exactly 12 g of carbon-12. Experimental measurements have determined that this number is very large:

    1 mol of items = 6.02214179 × 1023 items

    Understand that a mole means a specific number of items, just like a dozen means a certain number of items—twelve, in the case of a dozen. But a mole is a much, much larger number of items. These items can be atoms, or molecules, or eggs; however, in chemistry, we usually use the mole to refer to the amounts of atoms or molecules. Although the number of items in a mole is known to eight decimal places, it is usually fine to use only two or three decimal places in calculations. The numerical value of items in a mole is often called Avogadro's number (NA). Avogadro's number is also known as the Avogadro constant, after Amadeo Avogadro, an Italian chemist who first proposed its importance.

    Example \(\PageIndex{1}\):

    How many molecules are present in 2.76 mol of H2O? How many atoms is this?


    The definition of a mole is an equality that can be used to construct a conversion factor. Also, because we know that there are three atoms in each molecule of H2O, we can also determine the number of atoms in the sample.

    \[2.76\, \cancel{mol\, H_{2}O}\times \frac{6.022\times 10^{23}molecules\, H_{2}O}{\cancel{mol\, H_{2}O}}=1.66\times 10^{24}molecules\, H_{2}O \nonumber\nonumber \]

    To determine the total number of atoms, we have

    \[1.66\times 10^{24}\cancel{molecules\, H_{2}O}\times \frac{3\, atoms}{1\, molecule}=4.99\times 10^{24}\, atoms \nonumber\nonumber \]

    Exercise \(\PageIndex{1}\)

    How many molecules are present in 4.61 × 10−2 mol of \(\ce{O2}\)?


    2.78 × 1022 molecules

    How big is a mole? It is very large. Suppose you had a mole of dollar bills that need to be counted. If everyone on earth (about 6 billion people) counted one bill per second, it would take about 3.2 million years to count all the bills. A mole of sand would fill a cube about 32 km on a side. A mole of pennies stacked on top of each other would have about the same diameter as our galaxy, the Milky Way. Atoms and molecules are very tiny, so one mole of carbon atoms would make a cube that is 1.74 cm on a side, small enough to carry in your pocket. One mole of water molecules is approximately 18 mL or just under 4 teaspoons of water.

    Why is the mole unit so important? It represents the link between the microscopic and the macroscopic, especially in terms of mass. A mole of a substance has the same mass in grams as one unit (atom or molecules) has in atomic mass units. The mole unit allows us to express amounts of atoms and molecules in visible amounts that we can understand.

    For example, we already know that, by definition, a mole of carbon has a mass of exactly 12 g. This means that exactly 12 g of C has 6.022 × 1023 atoms:

    12 g C = 6.022 × 1023 atoms C

    We can use this equality as a conversion factor between the number of atoms of carbon and the number of grams of carbon. How many grams are there, say, in 1.50 × 1025 atoms of carbon? This is a one-step conversion:

    \[1.50\times 10^{25}\cancel{atoms\, C}\times \frac{12.0000\, g\, C}{6.022\times 10^{23}\cancel{atoms\, C}}=299\, g\, C\nonumber \]

    But it also goes beyond carbon. Previously we defined atomic and molecular masses as the number of atomic mass units per atom or molecule. Now we can do so in terms of grams. The atomic mass of an element is the number of grams in 1 mol of atoms of that element, while the molecular mass of a compound is the number of grams in 1 mol of molecules of that compound. Sometimes these masses are called molar masses to emphasize the fact that they are the mass for 1 mol of things. (The term molar is the adjective form of mole and has nothing to do with teeth.)

    Here are some examples. The mass of 1 hydrogen atom is 1.0079 u; the mass of 1 mol of hydrogen atoms is 1.0079 g. Elemental hydrogen exists as a diatomic molecule, H2. One molecule has a mass of 1.0079 u + 1.0079 u = 2.0158 u, while 1 mol of H2 has a mass of 1.0079 g + 1.0079 g = 2.0158 g. One molecule of H2O has a mass of about 18.01 u; 1 mol H2O has a mass of 18.01 g. A single unit of NaCl has a mass of 58.45 u; NaCl has a molar mass of 58.45 g. In each of these moles of substances, there are 6.022 × 1023 units: 6.022 × 1023 atoms of H, 6.022 × 1023 molecules of H2 and H2O, 6.022 × 1023 units of NaCl ions. These relationships give us plenty of opportunities to construct conversion factors for simple calculations.

    Example \(\PageIndex{2}\): Sugar

    What is the molar mass of sugar (\(\ce{C6H12O6}\))?


    To determine the molar mass, we simply add the atomic masses of the atoms in the molecular formula; but express the total in grams per mole, not atomic mass units. The masses of the atoms can be taken from the periodic table.

    6 C = 6 × 12.011 = 72.066
    12 H = 12 × 1.0079 = 12.0948
    6 O = 6 × 15.999 = 95.994
    TOTAL = 180.155 g/mol

    Per convention, the unit grams per mole is written as a fraction.

    Exercise \(\PageIndex{2}\)

    What is the molar mass of \(\ce{AgNO3}\)?


    169.87 g/mol


    The mole is a key unit in chemistry. The molar mass of a substance, in grams, is numerically equal to one atom's or molecule's mass in atomic mass units.

    6.1: The Mole and Avogadro’s Number is shared under a CC BY-NC-SA 3.0 license and was authored, remixed, and/or curated by LibreTexts.

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