Physical chemistry 1
Amount of substance
Relative atomic and molecular masses, the Avogadro constant and the mole
Relative atomic mass Ar
• The mass in grams of any atom or molecule is too small to find by weighing, so the masses of
atoms are compared, and relative masses are used.
• In the past hydrogen was given an Ar of 1. Now 1/12 or Carbon has an Ar of exactly 1.
• The relative atomic mass – Ar – is the weighted average of an atom of an element, taking into
account its naturally occurring isotopes, relative to 1/12 of the Ar of carbon-12.
• Ar = (average mass of 1 atom of an element *12) / (mass of 1 atom of carbon-12).
Relative molecular mas Mr
• Molecules can be handled in a same way, by comparing the mass of a molecule to that of 1/12
of carbon-12.
• You find the Mr by adding the Ar of every atom present in the molecule. E.g. H2O = (2*1)+16 =
18.
Relative formula mass
• This also has the symbol Mr but is used for ionic compounds as they don’t exist as molecules.
The Avogadro constant and the mole
• 1 atom of any element is too small to see with an optical microscope and impossible to weigh
individually.
• To count atoms, we weigh large numbers of them.
• If you weight out the Ar of any element, it will contain the same number of atoms.
• If you weigh out the Mr of a molecule, it will have the same number of molecules.
• If you weigh out the Mr of a compound (in grams) you have the number number of entities.
• Entities is a general word for a particle.
The Avogadro’s constant
• It’s the number of atoms in 12g carbon-12.
• 6.022x1023.
The mole
, Physical chemistry 1
Amount of substance
• The amount of substance that contains 6.022x1023 particles.
Mass
Mr * moles
• The Ar of any element (in grams) contains 1 mole of atoms.
• The Mr of a substance (in grams) contains 1 mole of entities.
• Number of moles = Mass * Mr.
Moles in solution
Solutions
• Consists of a solvent with a solute dissolved in it.
• To get a solution with a concentration of 1 mol dm-3, add solvent to 1 mole of solvent.
Units of concentration
• Concentration tells us how much solute is present in a known volume of solution.
• It’s measured in mol dm-3.
• Concentration (C) (mol dm-3) = no. of mole (n) / volume (V) (dm3)
Number of moles in a given solution.
• Number of moles in solution (n) = concentration (C) (mol dm-3) * volume (V) (dm3) / 1000
Ideal gas equation
• The volume of given mass isn’t fixed. It changes with pressure and temperature.
• There are several relationships for a given mass which link pressure, temperature and volume of
gas.
Boyle’s law
• The product of pressure and volume is constant as long as temperature remains constant.
• Pressure (p) x volume (V) = constant.
Amount of substance
Relative atomic and molecular masses, the Avogadro constant and the mole
Relative atomic mass Ar
• The mass in grams of any atom or molecule is too small to find by weighing, so the masses of
atoms are compared, and relative masses are used.
• In the past hydrogen was given an Ar of 1. Now 1/12 or Carbon has an Ar of exactly 1.
• The relative atomic mass – Ar – is the weighted average of an atom of an element, taking into
account its naturally occurring isotopes, relative to 1/12 of the Ar of carbon-12.
• Ar = (average mass of 1 atom of an element *12) / (mass of 1 atom of carbon-12).
Relative molecular mas Mr
• Molecules can be handled in a same way, by comparing the mass of a molecule to that of 1/12
of carbon-12.
• You find the Mr by adding the Ar of every atom present in the molecule. E.g. H2O = (2*1)+16 =
18.
Relative formula mass
• This also has the symbol Mr but is used for ionic compounds as they don’t exist as molecules.
The Avogadro constant and the mole
• 1 atom of any element is too small to see with an optical microscope and impossible to weigh
individually.
• To count atoms, we weigh large numbers of them.
• If you weight out the Ar of any element, it will contain the same number of atoms.
• If you weigh out the Mr of a molecule, it will have the same number of molecules.
• If you weigh out the Mr of a compound (in grams) you have the number number of entities.
• Entities is a general word for a particle.
The Avogadro’s constant
• It’s the number of atoms in 12g carbon-12.
• 6.022x1023.
The mole
, Physical chemistry 1
Amount of substance
• The amount of substance that contains 6.022x1023 particles.
Mass
Mr * moles
• The Ar of any element (in grams) contains 1 mole of atoms.
• The Mr of a substance (in grams) contains 1 mole of entities.
• Number of moles = Mass * Mr.
Moles in solution
Solutions
• Consists of a solvent with a solute dissolved in it.
• To get a solution with a concentration of 1 mol dm-3, add solvent to 1 mole of solvent.
Units of concentration
• Concentration tells us how much solute is present in a known volume of solution.
• It’s measured in mol dm-3.
• Concentration (C) (mol dm-3) = no. of mole (n) / volume (V) (dm3)
Number of moles in a given solution.
• Number of moles in solution (n) = concentration (C) (mol dm-3) * volume (V) (dm3) / 1000
Ideal gas equation
• The volume of given mass isn’t fixed. It changes with pressure and temperature.
• There are several relationships for a given mass which link pressure, temperature and volume of
gas.
Boyle’s law
• The product of pressure and volume is constant as long as temperature remains constant.
• Pressure (p) x volume (V) = constant.
