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Gram Formula Mass

The SI (systeme international) unit for “amount of a substance” is the mole. A mole is equal to 6.022 x 1023. Thus, a mole of pennies would be 6.022 x 1023 pennies. This number is very large and difficult to comprehend. A mole of soda cans would cover the surface of the earth to a depth of over 200 miles; a mole of unpopped popcorn kernels spread across the United States would be cover it to a depth of over 9 miles; if you counted atoms at a rate of 10 million per second, it would take about 2 billion years to count the atoms in one mole.

Chemicals are made up of molecules, ions and atoms. Since experimentally there are far too many of these representative particles (atoms, ions & molecules) to count individually, and they are too small, chemists count moles instead.

Although 6.022 x 1023 is often called Avogadro’s number, Amadeo Avogadro (1776-1856) never determined his number. Avogadro’s Law, postulated in 1811, was based on the work of Joseph Louis Gay-Lussac (whose work we will study later this year). Avogadro’s Law states that, so long as temperature and pressure are equal, an equal volume of any gas will contain the same number of molecules. Avogadro's Law was ignored until 1858, when Stanislao Cannizaro proved it experimentally by using it to determine accurate atomic weights.

Amadeo Avogadro
Avogadro's number can be determined experimentally by observing the actual mass (usually via the momentum) of a "single" atom or molecule, then comparing that to the mass of one gram-molecular-weight (or mole) of the same substance. One mole of any element is equal to its atomic weight in grams. For example, one mole of carbon would have a mass of 12.011 grams.

This tutorial shows how to calculate the gram formula mass of a compound. Gram formula mass (a.k.a. molar mass) is defined as the atomic mass of one mole of an element, molecular compound or ionic compound. The answer must always be written with the unit g/mol (grams per mole).

Calculating a substance's gram formula mass

Calculate the gram formula mass of ammonium phosphate.
Description of Action
1. Write the formula for the compound. (NH4)3PO4
2. Determine how many of each atom are in the compound. If there is a number outside the parenthesis, multiply each subscript by this number. If there is no subscript, assume it is one. N: 3
P: 1
3. Multiply the number of atoms by its atomic weight. (Every teacher is different but my students must round the atomic weight to the tenth’s place. N: 3 x 14.0 = 42.0
H:12 x 1.0 = 12.0
P: 1 x 31.0 = 31.0 (31.0 is rounded from 30.97)
O:4 x 16.0 = 64.0 (16.0 is rounded from 15.99)
4. Add your results and use the unit g/mol (grams per mole) on the end. 42.0 + 12.0 + 31.0 + 64.0 = 149.0 g/mol.

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