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Last updated: July 24, 2015
 
 
Chm-4
 
 
What is a Mol and How Is Molar Mass Used in Chemistry?
 
Posted: 19 July 2015  

What is a mol in chemistry? The textbook definition of a mol is virtually useless to a beginning chemistry student but it is given at the bottom of this page. A mol is a whole number that represents an astronomically huge number of molecules or atoms. In contrast a molecule or an atom is a singular unit whereas a mol represents a number of molecules or atoms that is much bigger than a trillion units.

Mols are the units that chemists used to figure out how many molecules are in a sample of a compound that is weighed in grams. The mol is the link between grams and molecules. Moles connect the macroscopic world of samples weighed in the laboratory to the microscopic world of atoms.

1 mol = Avogadro's number of units which is 602 followed by 21 zeros.

The word mol can be a bit confusing because it looks similar to molecule however as we already mentioned, they are not the same but they do have a relation to each other in the sense that the mol allows chemists to get a handle on how many molecules are in a measure of a substance. The astronomically huge number of units that equals one mole is called Avogadro’s Number.

602,000,000,000,000,000,000,000 molecules
Avogadro’s Number

Avogadro’s number is the number of molecules equal to 1 mol and it can be used as a conversion factor to see how many molecules are in a sample. In calculations with mols, scientific notation is used to write the extremely large number of molecules per 1 mol to avoid writing out 21 zeroes.

Atomic Mass Units (u) and grams/mol (molar mass)
On the periodic table each element has an atomic mass. The atomic masses for each element are found inside their elemental symbol box on the periodic table. For example, carbon’s atomic mass is 12.01, Nitrogen’s atomic mass is 14.01 and Calcium’s atomic mass is 40.08.

The unit for the atomic mass weight in grams is called the atomic mass unit, and the atomic mass unit, represented by a lower case “u” can be replaced with “grams/mol” when doing chemistry calculations.*

While one atom of carbon is 12.01 atomic mass units, it is also 12.01 grams/mols.

Conversion factors for atomic mass units of Carbon to moles
Carbon atomic mass unit per 1 mol 1 mol over 12.01 grams of carbon

Example:

Find how many grams are in 4 mol carbon?

4 mol carbon Multiplication sign Carbon atomic mass units per mole
=
48.04 g carbon

How many mols are in 250 grams of carbon?

250 g carbon Multiplication sign 1 mol per 12.01 grams of carbon atoms
=
20.8 mols carbon

How many atoms are in 20.8 moles of carbon?

20.8 mols carbon Multiplication sign Molecules over moles or Avogadro's Number over 1 mole.
=
Carbon atoms solution
Notice that you don't need to use any other conversion factor to find the number of molecules (or in this case atoms) from moles. All you do is multiply the number of mols by Avogadro's Number of units over 1 mol. Therefore, 20.8 moles of carbon has the same number units as 20.8 moles of citric acid or any other compound or element.

 

Citric acid
Citric Acid
chemical formula of citric acid

How To Find the Molecular Mass (g) and Molar Mass (g/mol)

Frequently you will need to find what the molecular mass or molar mass of a compound is. The molecular mass is the sum of all the atomic masses in the formula by however many atoms of each element are in the compound.

The molar mass is numerically the same as the molecular mass only the units are different. The units for molar mass are grams/mol and the units for molecular mass are grams.

 

What is the Molecular Mass of Citric Acid ?

Step 1 Multiply the atomic mass units of each element (found on the periodic table) by the number of atoms of the element in the formula.

carbon 12.01 g x 6 = 72.06 g

hydrogen 1.008 g x 8 = 8.064 g

oxygen 16.00 x 7 = 112 g

 

Step 2: Add the masses of each element in the formula found in step 1.

72.06 + 8.064 +112 = 192.12 g citric acid (molecular mass)

or

192.12 g/mol citric acid (molar mass).

 

How many grams of citric acid are in 2 mols of citric acid?

2 mol citric acid Multiplication sign Citric acid molar mass
=
384 g citric acid

 

Percent Composition

The percent composition is the percent of each element in a given chemical compound. When asked to find the percent composition of a compound, find the molecular mass, then divide the atomic mass of each element (multiplied by however many atoms of the element are in one molecule of the compound) by the molecular mass. There is a percent value for each element in the compound. And all of the percent values of the compound should add up to 100.00.

For a calcium fluoride molecule, because Calcium has a positive 2 charge and fluoride has a negative 1 charge, there would need to be 2 fluoride atoms to make the molecule electrically neutral.

The formula for calcium fluoride is Calcium flouride .

The molar mass of this molecule is: =

40.08 g Ca
+ 19.00(2) g F =
78.08 grams per mol of calcium flouride

 

The percentage composition of calcium fluoride:

Percent composition calcium in calcium fluoride Percent calcium in calcium fluoride = 51.33% calcium
Percent composition fluoride in calcium fluoride Percent fluoride in calcium fluoride = 48.67% fluoride

The percent composition of calcium fluoride is 51.33 g calcium and 48.67 g fluoride. You can check your answer by adding the two percent compositions together and they should equal 100 or 99.99.

 



by Andrea Boggs


Reference list


Cracolice & Peters. Introductory Chemistry. 5e. Cengage Learning. 2013.

"One mole is the amount of any substance that contains the same number of units as the number of atoms in 12 grams of Carbon-12." (p.184)

* “If we think of percent as the number of grams of one element per 100 g of the compound, then a 100-g sample must contain 85.6 g of carbon and 14.4 g of hydrogen. From this we see that percentage composition figures represent the grams of each element in a 100-g sample of the compound.” (p.195)

 

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