Relation between Mass and Molar Mass

a. Counting by Mole

A drop of water contains billions of billions ... of water molecules

To express the quantity of substance in chemistry, we use a quantity that contains many elements:

  • In chemistry, we count elements in moles (mol for short)
  • 1 mole contains 6.02x1023 entities
  • The quantity of substance is thus expressed in moles with the symbol n
  • This number 6.02x1023, is called Avogadro's number noted NA

Examples:

  • n(H2O) = 1 mole corresponds to 6.02x1023 water molecules
  • n(H2O) = 0.5 mole corresponds to 0.5x6.02x1023 water molecules, i.e., 3.01x1023 molecules.

If N is the number of elements counted per unit: N = n x NA

Note: in practice, we often use n without necessarily needing N,
but it is important to know that n represents a number of "packets".

b. From Mole to Mass

If we consider one of the chemical elements, such as oxygen(O), and weigh 1 mole of oxygen atoms,
we obtain what is called the atomic molar mass of oxygen.

The atomic molar mass of oxygen is always the same

You're right 😎! Since the number of atoms is fixed, the obtained mass is always the same.

This is a reference value found in the periodic table, an excerpt of which is shown below:
periodic table

The atomic molar mass is noted with the symbol M, so for oxygen: MO = 16.0 g/mol.

It is impossible to list the molar masses of molecules (referred to as molecular molar mass),
there are far too many.

We can easily calculate a molecular molar mass, noting that:
  • A molecule is an "assembly" of atoms
  • The molecular molar mass is therefore equal to the sum
    of the atomic molar masses of all the atoms that compose the molecule.

For example:
  • We know that MH = 1.0 g/mol and MO = 16.0 g/mol
  • So M(H2O) = 2 x MH + 1xMO
  • M(H2O) = 2x1 + 1x16 = 18 g/mol

Let's check if I understood correctly:

Knowing that MH = 1.0 g/mol, MC = 12.0 g/mol, and MO = 16.0 g/mol
We can deduce that MC2H6O = 46.0 g/mol

c. From Molar Mass to Mass.

sugar cube
Let's recap with the example of a sugar cube.
For this sugar cube, I can determine the following quantities:
  • n → quantity of substance in mol
  • M → molar mass in g/mol
  • m → mass in g
These quantities are related:
For 1 mole → m = M (by definition M represents the mass of one mole)
For n moles → m = n x M or n = m/M.

Let's summarize what we've just seen with the example below:

  • If we weigh a sugar cube, we find approximately 6g
  • The molecule composing sugar is sucrose with the formula C12H22O11

The molar mass of sugar is MC12H22O11 = 342g/mol and the quantity of substance in a sugar cube (6g) is n = 0.25 mol.

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Now, it's your turn to play! A mass or quantity of substance (random) to find.
Which compound would you prefer to work on?:

Relation between Mass, Molar Mass, and Quantity of Substance

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