Analía Bellizzi – Chemistry Classes

Ronald Reagan Senior High School

The Mole (Abbreviated: mol)

Basic concepts

  • The Mole, (mol) is the unit of amount of substance.
  • The mole is defined as the number of atoms in 12 grams of carbon-12.
  • The mole represents a huge number

1 mole = 602 000 000 000 000 000 000 000

1 mole = 6.02 × 1023 

This number is called Avogadro’s number , (or Avogadro’s constant), in honor of the Italian physicist Amadeo Avogadro. Avogadro did not calculate the number, but he proposed that , measured under the same conditions, equal volumes of gases contain the same number of molecules, which is the base of the mole concept.

12 grams of C-12 atoms contain exactly 1 mole of carbon atoms.  

In the same way that we can say one mole of carbon-12 atoms has 6.022 × 1023 atoms and a mass of 12 grams,  we can expand the concept to any element. 

The atomic mass (Ar) of any element expressed in grams contains 1 mol of atoms of that element.

ELEMENT Ar Molar Mass Moles Number of Atoms
C- Carbon
12
12 g
1 mol Carbon atoms
6.022 × 1023  Carbon atoms
Na- Sodium
23
23 g
1 mol Na atoms
6.022 × 1023  Sodium atoms

Notice that the Ar expressed in grams contains always 1 mole of atoms of the element. 

We can expand this concept to the whole periodic table, since:

    • 64 g of Copper contains 1 MOLE of atoms of Copper.
    • 32 g of Sulfur contains 1 MOLE of atoms of sulfur
    • 56 g of Iron contains 1 MOLE of atoms of Iron

Calculations – Worked examples. 

Calculate the molar mass of elements and compounds

In the case of 1 mole of an element, the information is taken from the periodic table and we do not need any calculation:

Example 1 – What is the molar mass of oxygen atoms?

Explanation: The question is asking for the mass of 1 mole. We know that the Ar of Oxygen is 16 AMU, so 1 mol of atoms of oxygen will have a mass of 16g

      • 1 mol O atoms =  16 g/mol

Example 2 – What is the molar mass of Oxygen gas?

Explanation: In the case of a molecule, we need to add up all the masses  from the periodic table of all the elements in the formula and express that in grams.

      • 1 mol O2 = 2x(16 g) = 32 g O2 /mol

Example 3 – what is the molar mass of ozone? O3

Explanation: In the case of a molecule, we need to add up all the masses  from the periodic table of all the elements in the formula and express that in grams.

        • 1 mol O3 = 3x(16 g) = 48 g O3 /mol

Example 4 – what is the molar mass of water? H2O

Explanation: In the case of a molecule, we need to add up all the masses  from the periodic table of all the elements in the formula and express that in grams.

        • 1 mol H2O = 2x(ArH) + (ArO) = 2 + 16 = 18 g H2O/mol

Example 5 – what is the molar mass of Sulfuric acid? H2SO4

Explanation: In the case of a molecule, we need to add up all the masses  from the periodic table of all the elements in the formula and express that in grams.

        • 1 mol H2SO4 = 2x(ArH) + (Ars) + 4x(ArO) = 2 +32 + 48 = 98 g H2SO4/mol

Calculating mass of elements and compounds from number of moles

We can use proportions or dimensional analysis (conversion factors) 

we know that: 

1 mole of an element = Ar of the element in grams

1 mole of a compound = Mr  of the element in grams

Example 1- MOLES TO MASS – Calculate the mass of 0.5 moles of H2SO4

1 . calculate the molar mass of the compound: 

1 mol H2SO4 = 2x(ArH) + (Ars) + 4x(ArO) = 2 +32 + 64 = 98 g H2SO4/mol

2. multiply the data given by a fraction that allows us to simplify the units. 

Example 2 –  MASS TO MOLES – Calculate the mass of 3.15 g of HNO3

1 . calculate the molar mass of the compound: 

1 mol HNO3 = ArH + (ArN) + 3x(ArO) = 1 +14 + 48 = 63 g HNO3/mol

2. multiply the data given by a fraction that allows us to simplify the units. 

 

Example 2 –  MASS TO MOLES – Calculate the mass of 3.15 g of HNO3

1 . calculate the molar mass of the compound: 

1 mol HNO3 = ArH + (ArN) + 3x(ArO) = 1 +14 + 48 = 63 g HNO3/mol

2. multiply the data given by a fraction that allows us to simplify the units. 

 

  1. Knowing that the relative atomic mass of the element expressed in grams is the MOLAR MASS (The mass of 1 mole), we can calculate the number of moles present in any mass of the substance.Example: How many moles of Sulfur are there in 64 g of that substance?

  1. Knowing that the relative atomic mass of the element expressed in grams is the MOLAR MASS (The mass of 1 mole), we can calculate the number of grams present in any amount of moles of the substance.Example: How many grams of copper are there in 1.2 moles of copper meta
 
  1. Note these equations are for either an element or a compound. The mass of one mole of a compound can be expressed as the Mr in grams. It represents 1 mole of molecules of that substance.
    so:
    • 18 grams of water contain 1 mole of molecules of water.
    • 44 grams of carbon dioxide contain 1 mole of molecules of carbon dioxide.
    • etc.

Here you have some videos to help you understand the concept.

 

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Knowing that the relative atomic mass of the element expressed in grams is the MOLAR MASS (The mass of 1 mole), we can calculate the number of moles present in any mass of the substance.

Example: How many moles of Sulfur are there in 64 g of that substance?