C3

• What is the relative formula mass (M)?

  It is the sum of all the relative atomic masses (A) of the atoms in the molecular formula.
• How do you calculate the percentage mass of an element in a compound?

  Percentage mass = \(\frac{A \times \text{number of atoms of that element}}{M} \times 100\)
• What is a balanced equation?

  A balanced equation has the same number of atoms of each element on both sides.
• What does the equation \(2\text{Mg} + \text{O}_2 \rightarrow 2\text{MgO}\) represent?

  It shows that there are 2 magnesium atoms and 2 oxygen atoms on each side.
• What does the conservation of mass state in a chemical reaction?

  No atoms are created or destroyed, so the total masses of reactants and products are the same.
• What is one mole equal to?

  One mole is equal to \(6.02 \times 10^{23}\) particles of a substance.
• What types of particles can one mole represent?

  One mole can represent atoms, molecules, ions, or electrons.
• How is the mass in grams of one mole of atoms of an element defined?

  It is equal to the relative atomic mass (A) of the element.
• How is the mass in grams of one mole of molecules of a compound defined?

  It is equal to the relative formula mass (M) of the compound.
• What happens to the mass in a reaction if a gas is produced and escapes?

  There is a decrease in mass because the gas is no longer accounted for.
• What is an example of a reaction that produces a gas and results in a decrease in mass?

  The thermal decomposition of calcium carbonate produces CO₂ gas.
• What happens to the mass in a reaction if a gas from the air is a reactant?

  There is an increase in mass because the gas adds to the mass in the vessel.
• What is an example of a reaction that results in an increase in mass?

  The reaction of magnesium with oxygen produces a solid and consumes O₂ gas.
• How do balanced equations relate to the number of moles of substances in a reaction?

  Balanced equations indicate how many moles of each substance participate in the reaction.
• In the equation \( \text{Mg} + 2\text{HCl} \rightarrow \text{MgCl}_2 + \text{H}_2 \), how many moles of HCl react with one mole of Mg?

  Two moles of HCl react with one mole of Mg.
• What is the first step in balancing equations using masses?

  Divide the mass by the relative formula mass (Mᵣ) to find the number of moles of each substance.
• What do you do if the results of dividing the number of moles by the smallest number of moles are not whole numbers?

  Multiply them by the same number so that they are whole numbers.
• What is a limiting reactant?

  A limiting reactant is one that gets completely used up in a reaction, limiting the amount of product formed.
• How can you determine the mass of a product from the mass of the limiting reactant?

  Divide the mass of the limiting reactant by its Mᵣ to find the number of moles, then use the balanced equation to find the moles of the product.
• What is the volume occupied by one mole of any gas at room temperature and pressure?

  One mole of any gas occupies 24 dm³ at room temperature and pressure.
• How is concentration defined?

  Concentration is the amount of substance dissolved in a certain volume of solution.
• What happens to concentration when the amount of solute increases?

  The concentration increases.
• What happens to concentration when the volume of solvent increases?

  The concentration decreases.
• What are the two ways to measure concentration?

  1. Concentration (g/dm³) = mass of solute / volume of solvent
  2. Concentration (mol/dm³) = number of moles of solute / volume of solvent
• How do you find an unknown concentration from a titration result?

  Use the known concentration and measured volume of reactant 1 to find moles, then use the reaction equation to find moles of reactant 2.
• What is the purpose of repeating titration experiments?

  To find the uncertainty in the mean value of the results.
• What is atom economy?

  Atom economy is the percentage of the mass of reactants that ends up as useful products.
• What are three advantages of using reactions with higher atom economies?

  1. Use up resources at a slower rate. 2. Don't produce a lot of waste. 3. More profitable.
• What is percentage yield?

  Percentage yield is a comparison of the amount of product actually made with the maximum theoretical amount of product.
• How do you calculate percentage yield?

  Percentage yield = \(\frac{\text{mass of product actually made}}{\text{maximum theoretical mass of product}} \times 100\)
• What are three common reasons for yields being lower than 100%?

  1. Reaction is not completed (e.g., reversible).
  2. Side reactions use up some reactants or products.
  3. Some product is lost when separated from the reaction mixture.