Formulae, Equations and Amounts of Substance


Chemical Quantities and Formulae

  • An element is a substance that cannot be broken down by chemical means into other substances. 
  • The atomic number, Z, is the number of protons=number of electrons of an atom.
  • An atom is the smallest part of an element that can take part in a chemical change. Atoms have no charge (neutral).
  • The mass number, A, of an atom is the number of protons + number of neutrons in that atom.
  • Isotopes are atoms of an element with the same number of protons but different number of neutrons.
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Chemical Quantities and Formulae (cont)

  • The relative atomic mass of an element is the average mass of all the isotopes of that element relative to 1/12 of the mass of a Carbon 12 atom.
  • A compound is formed when 2 or more elements are chemically bonded together.
  • A molecule is the smallest part of a covalent compound that can exist on its own.
  • Ions are charged particles.
  • An ion is formed when an atom gains or loses one or more electrons
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Chemical Quantities and Formulae (cont)

  • The relative molecular mass of a compound is the sum of the relative atomic masses of all the atoms in that compound (also called relative formula mass).
  • The mole is the unit of a chemical compound, or amount of substance.
  • 1 mole is the amount of substance that contains as many particles as there are atoms in 12g Carbon 12.
  • The mass of one mol of substance is called the molar mass
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Chemical Quantities and Formulae (cont)

  • Number of moles = mass/molar mass
  • Number of moles= concentration x volume
  • Concentration can be expressed as parts per million.
  • ppm= number of parts of chemical/number of parts it is containted in x 1 million
  • The emperical formula of a compound shows the simplest atom ratio of the elements in that compound.
  • The molecular formula of a compound shows how many atoms/ions of each element combine to make that compound.
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Chemical Equations and Reacting Masses

  • A balanced chemical equation tells us what is reacting and what is being produced, and also the proportions of the atoms involved.
  • Ions that appear on both sides of an equation in the same state are called spectator ions. They take no part in the reaction and can be removed from the full equation to give an ionic equation.
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Reactions with Gases

  • The molar volume of a gas is the volume of 1 mole of that gas.
  • For all gases this is 24dm3 at standard temperature and pressure (25'C/198K and 1atm).
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Percentage Yield and Atom Economy

  • No atoms are gained or lost during a chemical reaction. However, you are unlikely to obtain the theoretical maximum amount of product because:
  • The reaction may be reversable
  • Some reactants or products may be left behind in the apparatus e.g during filtering or pouring.
  • Some of the reactants may react differently to the expected reaction (side reactions, forming bi-products). 
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Percentage Yield and Atom Economy (cont)

  • The amount of product obtained is the yield
  • When this is compared to the maximum theoretical yield as a percentage, we get a percentage yield.
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Percentage Yield and Atom Economy

  • Salts are ionic compounds. A double salt contains more than one anion or cation.
  • Salts can be produced by neutralising acids with an alkali, or with a metal/metal hydorxide/metal oxide/metal carbonate. 
  • A soluble salt must be crystalised from a saturated solution- concentrate the solution by driving off some of the water (bunsen burner), leaving to evaporate and then filtering.
  • An insoluble salt forms a precipitate and can be filtered off, washed and dried
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Percentage Yield and Atom Economy (cont)

  • Atom economy is a measure of the amount of starting material that ends up as useful product- so it is not the same as the yield.
  • Atom economy is calculated using a balanced chemical equation, assuming it has 100% yield.
  • Atom economy= molar mass of required product/total molar mass of reactants
  • A high atom economy reduces the amount of waste product and increases the amount of useful product.
  • Atom economy can be improved by finding use for bi-products.
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