Amount of substance

• Chemicals are usually measured by mass or volume. Because reactions take place on an atomic scale, chemists need a method for converting a measured mass or volume into the actual number of particles involved in reactions
• Chemists use a quantity called amount of substance, n to count the number of particles in a substance, measure in a unit called the mole, mol
• One mole is the amount of substance that contains 6.02x 10 23 particles
• The Avogadro constant, NA is 6.02 x 10^23 mol-1

  the number of particles in each mole of carbon-12
• The choice of 6.02x10^23 particles per mole is directly linked to the mass of carbon-12, the standard for the measurement of relative atomic masses.
• 12g of carbon-12 contains 6.02x10^23 atoms
• You can easily find the mass of one mole of atoms of any element - it is the relative atomic mass in grams
• one mole of carbon atoms has a mass of 12g

  one mole of hydrogen atoms has a mass of 1g

  one mole of magnesium atoms has a mass of 24.3g

  one mole of iron atoms has a mass of 55.8g
• Molar mass: M, gives a convenient way of linking moles with mass for any chemical substance
• molar mass gives the mass in grams in each mole of the substance, molar mass is the mass per mole of a substance, the units of molar mass are g mol-1
• amount of substance,n = mass, m / molar mass, M
• Some compounds are made up of small units called molecules - two or more atoms held together by covalent bonds
• The molecular formula is the number of atoms of each element in a molecule
• The empirical formula is the simplest whole-number ratio of atoms of each element in a compound
• The empirical formula is important for substances that do not exist as molecules. This includes metals, some non-metals (e.g. carbon and silicon) and ionic compounds - these substances form giant crystalline structures of atoms or ions
• It would be impossible to base a formula on the actual number of atoms or ions - the numbers would go into billions and would vary depending on the size of the crystal
• The empirical formula is a ratio of atoms or ions in the structure and will always be the same
• Relative molecular mass (Mr) compares the mass of a molecule with the mass of an atom of carbon-12
• You can easily calculate a relative molecular mass by adding together the relative atomic masses of the elements making up the molecule
• Relative formula mass compares the mass of a formula unit with the mass of an atom of cerbon-12. It is calculated by adding together the relative masses of the elements in the empirical formula
• You can predict the formula of an ionic compound from its ions, but if you do not know which ions are in a compound, the formula can be worked out from the results of experiments.
• Investigation the chemical composition of a substance is called analysis
• Many coloured crystals are hydrated - water molecules are part of their crystalline structure
• This water is known was water of crystallisation.
• When blue crystals of hydrated copper (II) sulfate are heated, bonds holding the water within the crystal are broken and the water is driven off, leaving behind white anhydrous copper (II) sulfate.