formulae and equations

• What is the formula of a compound?

  A set of symbols and numbers indicating the elements present and their ratios.
• What do the symbols in a compound's formula represent?

  The elements that are present in the compound.
• What do the numbers in a compound's formula indicate?

  The ratio of the numbers of atoms of the different elements in the compound.
• What is the formula for sulfuric acid?


  H₂SO₄
• How do you represent multiple molecules in a chemical formula?

  By writing the appropriate number in front of the formula.
• If you have 3HNO₃, how many atoms of nitrogen are present?

  3 atoms of nitrogen.
• What is the requirement for the total number of charges in an ionic compound?

  The total number of positive charges must equal the total number of negative charges.
• What are the steps to calculate the formula for ionic compounds?

  1. Write the symbols of the ions in the compound.
  2. Balance the ions so that the total charge is zero.
  3. Write the formula without charges, using small numbers for the number of ions.
• What is the formula for sodium oxide?

  Na₂O
• How many sodium ions are needed to balance one oxide ion in sodium oxide?

  Two sodium ions.
• What is the formula for calcium hydroxide?

  Ca(OH)₂
• What is the oxidation number of an uncombined element?

  Zero.
• What does the oxidation number indicate?

  The number of electrons that need to be added or removed to make an element neutral.
• What is the oxidation number of magnesium in Mg²⁺?

  +2
• What is the oxidation number of chloride in Cl-

  -1
• What are the rules for assigning oxidation numbers?


  • Uncombined elements have an oxidation number of 0.
  • The sum of oxidation numbers in a compound is 0.
  • In an ion, the sum equals the overall charge.
  • Group 1 metals have +1, Group 2 metals have +2.
  • Oxygen is usually -2, except with fluorine or in peroxides.
  • Hydrogen is +1, except in metal hydrides.
  • The most electronegative element gets the negative oxidation number.
• What is the oxidation number of manganese in MnO₄⁻?

  +7
• What are the state symbols used in chemical equations?


  • (s) for solid
  • (l) for liquid
  • (g) for gas
  • (aq) for aqueous solution
• What must be true for a chemical equation to be balanced?

  The same number of atoms must be present on each side of the equation.
• How do you balance a chemical equation?

  By multiplying a formula by putting a number in front of it.
• What is the difference between a chemical equation and an ionic equation?

  An ionic equation focuses on the changes taking place, excluding spectator ions.
• What happens when barium chloride is mixed with sodium sulfate?

  A white precipitate of barium sulfate forms.
• What is a spectator ion?

  • An ion that does not change during a reaction.
  • It is excluded from the ionic equation.
• What is the chemical equation for the reaction between barium chloride and sodium sulfate?

  BaCl₂(aq) + Na₂SO₄(aq) → BaSO₄(s) + 2NaCl(aq)
• What is the purpose of writing ionic equations?

  To show the changes taking place in a reaction while excluding spectator ions.
• What is the formula for barium sulfate?


  BaSO₄