The Mole

Once we know the mass of a sample we can use the mass of the atoms or molecules in the sample to determine how many atoms or molecules are in the sample.

How many 12C atoms are in a 1.00 kg block of pure 12C isotope?

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Obviously atoms and molecules are not a convenient unit of measure when we're working with macroscopic (i.e., human size) objects. For this reason chemist define a new unit of measure called the mole.

1 mole is defined as the number of carbon atoms in exactly 12.000000 grams of pure 12C.

From our definition of the atomic mass unit we get

1 mole = 6.022140857 x 1023 chemical units

The number 6.022140857 x 1023 is called Avogadro's number.

How many moles are in a 1.00 kg block of pure 12C isotope?

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Of course we know a 1.00 kg block of naturally occuring carbon will contain a mixture of 12C, 13C, and even some 14C isotopes. So the number of moles of carbon atoms in a 1.00 kg block of naturally occuring carbon is

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We can also calculate the mass of a mole of molecules.

What is the mass of a mole of methane (CH4) molecules?

mass of 1 mole of C is12.011 g
mass of 4 moles of H is4 X 1.00g

mass of 1 mole of CH4 is

16.043 g

In other words the molecular weight of CH4 is 16.043g.

Molecular weight = the mass in grams of 1 mole of a molecule.

For ionic compounds, which do not exist as individual molecules we use the term Formula weight

Formula weight = The mass in grams of 1 mole of the chemical formula.

What is the formula weight of CaCO3?

mass of 1 mole of Ca is 40.08 g
mass of 1 mole of C is 12.011 g
mass of 3 moles of O is 3 X 15.999 g

Formula weight of CaCO3 is

100.09 g

Let's look at another example.

How many hydrogen atoms are there in 2.50 g of NH3?

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Now, let's try some sample quiz questions on:

Avogadro's Number
  • Molecular Weight from Weight of Molecule:
  • Mass of Molecule:
  • Atoms Present in Mass of a Sample:
  • Atoms in a Shaped Material Given Density:
  • Molecules Present in Mass of a Sample:
The Mole Concept
  • Moles of One Type of Atom in a Mass of a Compound:
  • Total Number of Moles of Atoms in a Compound:
  • Comparison of Different Samples:
  • Moles of Water in Hydrates:
  • Grams of Sample from Number of Moles:
  • Moles from Grams of Sample:
  • Comparison of Moles of Atoms in Compounds:
  • Moles from Mass and Shape of An Object:
  • Molecular Weight Calculated from Molecular Formula:
  • Comparison of Molecular Weights of Several Compounds:

Homework from Chemisty, The Central Science, 10th Ed.

3.21, 3.23, 3.25, 3.29, 3.31, 3.33, 3.35, 3.37, 3.39, 3.41, 3.43, 3.45, 3.47, 3.49