This is class notes for the first year/AS chemistry. It covers the second chapter of physical chemistry - amount of substance including:
Relative atomic and molecular masses, the Avogadro's constant and the mole
Moles in solution
The ideal gas equation
Empirical and molecular formulae
Balanced...
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.
• 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.
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