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Mole, atomic weight, formula weight, molar mass

Definition

A mole (abbreviation: mol) is an amount of substance that contains as many objects (molecules, particles, ions, cells etc.) as the number of atoms in exactly 12 grams of ¹²C (carbon).

This number can be determined experimentally to be 6.022 x 10²³ (602,200,000,000,000,000,000,000), a number which is called Avogadro's number. The mole is simply a convenient unit for dealing with large numbers just like it is easier to talk about the weight of a ship in tons rather than ounces.

"Mole" has several other possible meanings, of course, but none of these generally applies to material safety data sheets.

Additional Info

A mole of anything has Avogadro's number of objects in it. So a mole of water (H2O) has 6.022 x 1023 water molecules, a mole of carbon atoms has 6.022 x 1023 carbon atoms and a mole of automobiles (rather unlikely) would have 6.022 x 1023 automobiles. Clearly, if the objects we are talking about have different masses (weight), then a mole of one substance will also have a different mass than the other. So while a mole of 12C (carbon) atoms will weigh 12 grams, a mole of 197Au (gold) atoms will weight 197 grams and a mole of sodium chloride (NaCl, table salt) will weigh 58.45 grams. How were we able to tell you that? Using these terms and definitions:

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• The IUPAC definition of atomic mass unit (amu) sets the mass of one ¹²C atom equal to 12 amu. We can determine the amu experimentally to be 1.66 x 10^-24 (0.00000000000000000000000166) grams. Just as Avogadro's number is very large, an amu is very small.

  Therefore, the atomic weight of a ¹²C atom is 12 amu. ¹⁴N atom has an atomic weight of 14 amu. You can look these values up in any chemistry textbook or on the web at WebElements.

• The formula weight of a substance is the sum of the atomic weights of each atom in the chemical formula. Therefore, the formula weight of benzene (C₆H₆) is equal to the mass of six carbon atoms (6 * 12 amu = 72 amu) plus the mass of six hydrogen atoms (6 * 1 amu = 6 amu) = 78 amu. Formula weight is commonly abbreviated FW

• When the chemical formula equals the molecular formula (see same link), then the formula weight is also called the molecular weight. Therefore, as the chemical and molecular formulas of benzene are both C₆H₆, the molecular weight of benzene is also 78 amu. The abbreviation commonly used for molecular weight is MW.

• The molar mass is the mass in grams of 1 mole of the substance. Molar masses (in grams) are therefore numerically equal to formula weights (in amu). In the example we've been using, this tells us that because the FW of benzene is 78 amu, then one mole of benzene must weigh 78 grams. Likewise, one mole of water (FW = 18 amu) has a molar mass of 18 grams.

MSDS Relevance

Moles are used to calculate molarity, a unit of concentration most often used for liquid solutions. The formula and molecular weights are also included on MSDS's in case one needs to calculate a molarity, for example.

Further Reading

• The Mole Concept at Indiana University Northwest.
• What Is a Mole? at DigitalFire Corporation.
• The Mole Concept at General Chemistry Online reviews how the mole is used in fundamental chemistry.
• WolframAlpha is a computational search engine that can return all kinds of chemical data, including mole calculations for various materials.
• Science geeks get a kick out calling October 23rd (10/23) "mole day"; visit the National Mole Day Foundation, Inc. for more info and fun.

See also: Chemical formula, IUPAC, mass units, molarity.

Additional definitions from Google and OneLook.

Disclaimer: The information contained herein is believed to be true and accurate, however ILPI makes no guarantees concerning the veracity of any statement. Use of any information on this page is at the reader's own risk. ILPI strongly encourages the reader to consult the appropriate local, state and federal agencies concerning the matters discussed herein.