C3

• What is needed to calculate the relative formula mass (M_r) of a compound?

  Relative atomic masses (A_r) of elements
• What is the A_r of chlorine?

  35.5
• How do you calculate the relative formula mass of a compound?

  Multiply A_r values by atom counts, then add
• What is the A_r of hydrogen?


  1
• What is the total M_r of hydrochloric acid (HCl)?

  36.5
• What are the A_r values for sulfur and oxygen in sulfuric acid (H₂SO₄)?

  A_r of sulfur = 32, oxygen = 16
• What is the total M_r of sulfuric acid (H₂SO₄)?

  98
• Why might substances appear to lose or gain mass in chemical reactions?

  Gases may escape into the environment
• What gas is released when wood is burnt?

  Carbon dioxide
• Why is it difficult to weigh gases at the end of a reaction?

  Gases can escape into the atmosphere
• How does the mass of the oxide produced in metal corrosion compare to the starting metals?

  It is larger due to added oxygen
• What happens to the mass of zinc and hydrochloric acid during their reaction?

  The mass decreases due to escaping hydrogen gas
• What is the precision of a digital thermometer?

  ± 0.1°C
• What does a reading of 35.4°C on a thermometer imply about the actual temperature?

  It could be between 35.3°C and 35.5°C
• Why is it important to repeat measurements in experiments?

  To reduce uncertainty and calculate an average
• What must be true for a chemical equation to be balanced?

  Same number of atoms of each element
• What is the unbalanced equation for the reaction of hydrogen with oxygen to make water?

  H₂ + O₂ → H₂O
• Why is the equation H₂ + O₂ → 2H₂O still unbalanced?

  More hydrogen atoms on the right side
• What is the balanced equation for the reaction of hydrogen and oxygen to form water?

  2H₂ + O₂ → 2H₂O
• What does the molecular formula represent?

  Actual amount of atoms in a molecule
• What is the molecular formula of glucose?

  C₆H₁₂O₆
• What does the empirical formula show?

  Simplest ratio of atoms in a molecule
• What is the empirical formula of glucose?

  CH₂O
• What are moles a measurement of?

  Chemical amounts
• Why is it inconvenient to talk about individual atoms?

  Atoms and molecules are very small
• How many atoms are in 1 mole of a substance?

  6.02 x 10²³
• What is the Avogadro constant?

  6.02 x 10²³ atoms
• What is a limiting reactant?

  The reactant completely used up in a reaction
• Why is it important to use up a reactant completely in a reaction?

  To ensure no reactant remains after the reaction
• What is achieved by using an excess of the other reactant in neutralizing an acid?

  No acid remains after the reaction
• What is the relative formula mass (M_r) of a substance?

  • Mass of 1 mole of that substance (in grams)
  • For an element, same as relative atomic mass
• How do you calculate the M_r of carbon dioxide?

  1. A_r of carbon = 12
  2. A_r of oxygen = 16
  3. M_r = 12 + (2 × 16) = 44