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Definitions of a mole

Key Concepts

  • Mole is abbreviated to mol and given the symbol n

  • 1 mole contains the same number of particles as there are in 12g of carbon-12 atoms by definition.
    This number is called the Avogadro number or Avogadro constant (NA or L) and is equal to 6.022 x 1023 particles.

  • 1 mole of a pure substance has a mass in grams equal to its relative molecular mass (Mr) (also known as molecular weight or formula mass or formula weight).
    This is known as the molar mass and is given the symbol M
    The units for molar mass are g mol-1 *

  • 1 mole of a gas occupies a specific volume at a particular temperature and pressure.
    This is known as the molar volume and given the symbol Vm
        Standard Temperature and Pressure (STP) is defined as a temperature of 0oC (273.15 K) and a pressure of 100 kPa (0.987 atm)#
        At STP 1 mole of an ideal gas has a volume of 22.71 L (Vm = 22.71 L mol-1)

        At 25oC (298.15 K) and 100 kPa (0.987 atm) 1 mole of an ideal gas has a volume of 24.79 L (Vm = 24.79 L mol-1)**

Examples

Avogadro Number (NA)

1 mole of particles contains the Avogadro Number, NA, of particles.
NA = 6.022 x 1023 particles per mole
  • 1 mole of helium atoms contains 6.022 x 1023 helium atoms.
  • 1 mole of carbon monoxide molecules contains 6.022 x 1023 carbon monoxide molecules.
  • 1 mole of H2O contains 6.022 x 1023 H2O molecules.
  • 1 mole of NaCl contains 6.022 x 1023 NaCl units.

Molar Mass (M)

1 mole of a pure substance has a mass in grams equal to its relative molecular mass (Mr).
  • 1 mole of helium gas (a monatomic gas with the formula He)
        chemical formula of helium gas is He
        relative molecular mass He = 4.003
        1 mole of He has a mass equal to its relative atomic mass in grams = 4.003 g
        Molar mass of helium = 4.003 g mol-1
        M(He) = 4.003 g mol-1

  • 1 mole of carbon monoxide gas
        chemical formula of carbon monoxide is CO
        relative molecular mass of CO = 12.01 + 16.00 = 28.01
        1 mole of CO has a mass equal to its relative atomic mass in grams = 28.01 g
        Molar mass of CO = 28.01 g mol-1
        M(CO) = 28.01 g mol-1

  • 1 mole of H2O
        relative molecular mass H2O = (2 x 1.008) + 16.00 = 18.016
        1 mole of H2O has a mass equal to its relative atomic mass in grams = 18.016 g
        Molar mass of H2O = 18.016 g mol-1
        M(H2O) = 18.016 g mol-1

  • 1 mole of NaCl
        relative molecular mass NaCl = 22.99 + 35.45 = 58.44
        1 mole of NaCl has a mass equal to its relative atomic mass in grams = 58.44 g
        Molar mass of NaCl = 58.44 g mol-1
        M(NaCl) = 58.44 g mol-1

Ideal Gas Volumes

At standard temperature and pressure (STP), defined as a temperature of 0oC (273.15 K) and a pressure of 100 kPa (0.987 atm), an ideal gas has a volume of 22.71 L.
  • 1 mole of helium gas at STP has a volume of 22.71 L
        Molar volume of helium gas is 22.71 L
        Vm(He(g)) = 22.71 L

  • 1 mole of carbon monoxide gas at STP has a volume of 22.71 L
        Molar volume of carbon monoxide gas is 22.71 L
        Vm(CO(g)) = 22.71 L

  • 1 mole of H2O(g) at STP has a volume of 22.71 L
        Molar volume of H2O(g) is 22.71 L
        Vm(H2O(g)) = 22.71 L
Practice Questions Homework Checker
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chemical formula:
Click Calculate:
For 1 mole of this substance:
Molar Mass = g mol-1
No. molecules = molecules
Gas volume = L at STP
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click Reset:

* The SI units for molar mass are actually kg mol-1 but Chemists in the laboratory work with much smaller amounts so usually use g/mol.
# Prior to 1982, standard temperature and pressure were defined as 0oC (273.15 K) and 1 atm (101.3 kPa), so 1 mole of gas would occupy 22.41 L
Similarly, at 25oC (298.15 K) and 1 atm (101.3 kPa), 1 mole of gas occupies 24.47 L.
**The conditions 25oC and 100 kPa are sometimes referred to as standard ambient temperature and pressure (SATP) or as standard laboratory conditions (SLC)

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