Revision Questions for Topic 3

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

  By summing the atomic masses of its elements
• What is the value of the Avogadro constant?

  6.022 x 10²³ mol⁻¹
• What formula relates the number of moles of a substance to its mass and relative formula mass?

  n = m / M
• What does conservation of mass mean?

  Mass is neither created nor destroyed in reactions
• Why might the mass of a reaction vessel decrease during a reaction?

  Because a gas may escape from the vessel
• How can you determine the number of moles of each substance that would react from a balanced reaction equation?

  By using the coefficients in the balanced equation
• What is meant by the term 'limiting reactant'?

  The reactant that is completely consumed first
• What volume does one mole of any gas occupy at room temperature and pressure?

  24 dm³
• What are the values of room temperature and pressure typically used in calculations?

  20°C and 1 atm
• What is concentration?

  The amount of solute in a given volume of solution
• What is the equation for calculating the concentration of a solution in g/dm³?

  C = m / V
• What is the equation for calculating the concentration of a solution in mol/dm³?

  C = n / V
• How would you convert concentration in mol/dm³ to g/dm³?

  Multiply by the molar mass of the solute
• What is the equation for calculating the atom economy of a reaction?

  Atom economy = (mass of desired product / total mass of reactants) x 100%
• Why is it better to use reactions that have a high atom economy?

  They produce less waste and are more efficient
• What is the atom economy of a reaction that produces one product from two reactants?

  50% if both reactants are used
• Why is the percentage yield of a reaction always less than 100%?

  Because of losses during the reaction and measurement
• What is the relationship between percentage yield and theoretical yield?

  Percentage yield = (actual yield / theoretical yield) x 100%
• Why is it always less than 100%?

  Because of practical losses and inefficiencies
• What is the maximum percentage yield possible in a reaction?

  100%
• Why is it better to use reactions with high percentage yields?

  They are more cost-effective and efficient
• Why is the percentage yield always less than 100%?

  Because of losses during the reaction process