Chemistry, 4: Stoichiometry

• When elements chemically combine, they form compounds that can be represented by chemical formulae.
• The combining power tells you how many bonds an atom can make with another atom or how many electrons its atoms will gain/lose/share to form a compound.
• The combining power of group I is 1
• The combining power of group II is 2
• The combining power of group III is 3
• The combining power of group IV is 4
• The combining power of group V is 3-
• The combining power of group VI is 2-
• The combining power of group VII is 1-
• Hydrogen has a combining power of 1
• To work out chemical formula of a simple compound:

  1. Write down the name of the compound
  2. Write down the chemical symbol for the elements in the compound
  3. Find combining power of each element
  4. If number cancels out, do so
  5. Swap combining power
  6. If number is 1, don’t write it down
• Elements bonded together is a radical

  E.g. Copper (II) sulfate, the sulfate is a radical
• What are radicals with the combining power of -1?


  Hydroxide (OH-)

  Hydrogen-carbonate (HCO3^-)

  Nitrate (NO3^-)

  Ammonium (NH4^+)
• What are radicals with the combining power of 2?

  Carbonate (CO3^2-)

  Sulfate (SO4^3-)
• What are radicals with the combining power of 3?


  Phosphate (PO4^3-)
• If formulas contains more than one radical unit, put it in brackets

  E.g. Calcium + hydroxide

  Ca = 2 and OH = 1

  Ca(OH)2
• Formula of an ionic compound can be worked out from ions present

  E.g. Sodium + chloride

  Sodium = 1+ and chlorine = 1-, cancels out

  NaCl

  Lead (II) + bromide

  Lead = 2+ and Bromine = 1-, does not cancel out and needs 2 of 1- charge to cancel out

  PbBr2
• The reactants are the starting chemicals
• The products are the finishing chemicals
• To write a chemical equation:

  1. Write down the word equation
  2. Write down the symbols for elements and formulae for compounds
  3. Balance the equation to make sure there are same amount of atoms on each side
  4. Include state symbols
• Some elements are diatomic, existing as 2 atoms
• What are the diatomic elements?

  • Hydrogen
  • Oxygen
  • Nitrogen
  • Chlorine
  • Bromine
  • Iodine
• Ionic equations show reactions involving ions (atoms or radicals that have gained or lost electrons)

  In ionic reactions, ions play no part and are spectator ions
• To find Mr, add up the mass of all electrons
• Mole is the amount of substance
• Avogrado constant is one mole of atoms, which is 6 x 10^23 atoms
• Relative atomic mass tells you the mass of a mole of atoms in an element

  E.g. A mole of carbon has a mass of 12g
• What is the formula for moles?


  Moles = mass / relative atomic mass (Mr)
• Chemical equations allows quantities of reactants and products to be linked together

  It tells you how much of the product you can expect to make from a fixed amount of reactants

  In a balanced equation, number in front of symbol or formula indicates number of moles present

  Number of moles can be converted into mass in grams
• A solution is made when a solute dissolves in a solvent
• The concentration of a solution depends on how much a solute is dissolved in how much solvent
• How do you find moles in solution?


  Moles = concentration (mol/dm3) * volume (dm3)
• 1dm3 = 1000cm3
• What is the equation to find moles in gases?


  Moles = volume / 24,000cm3

  OR 24dm3
• In reactions involving gas, it is better to use volume than mass

  One mole of gas occupies same volume under conditions of temperature and pressure