C3 - Quantitative Chemistry

• What is the formula for relative formula mass (M_r)?

  M_r = sum of all the relative atomic masses (A_r) of the atoms in the molecular formula
• How do you calculate the percentage mass of an element in a compound?

  Percentage mass = (A_r x number of atoms of that element) / M_r of the compound × 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 2Mg + O₂ → 2MgO illustrate?

  It illustrates 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 = $6.02 \times 10^{23}$ particles of a substance
• What is the mass in grams of one mole of atoms of an element equal to?

  The mass in grams of one mole of atoms is equal to the A_r of the element
• How do you calculate the number of moles (mol)?

  Number of moles = mass in g / M_r
• What happens to the mass in an unsealed reaction vessel if a gas escapes during a reaction?

  There is a decrease in mass because the gas is no longer accounted for
• What is the effect on mass in a reaction vessel if a gas from the air is a reactant?

  There is an increase in mass because the gas is added to the mass in the vessel
• What does the balanced equation Mg + 2HCl → MgCl₂ + H₂ indicate about the moles of substances involved?

  1 mole of Mg reacts with 2 moles of HCl to produce 1 mole of H₂ and 1 mole of MgCl₂
• How do you balance equations using masses?

  Divide mass by M_r to find moles, then divide by the smallest number of moles
• 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 a limiting reactant?

  Write a balanced equation, find moles of the limiting reactant, then use the equation to find 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 does concentration relate to the amount of solute and volume of solvent?

  Increasing the amount of solute increases concentration, while increasing the volume of solvent decreases concentration
• What are the two ways to measure concentration?

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

  Use the known concentration and measured volume of one reactant to find moles, then use the reaction equation to find moles of the other reactant
• What is atom economy and its formula?

  • Atom economy = (total M_r of desired products / total M_r of all reactants) × 100
  • It measures the percentage of reactants that become useful products.
• What are the advantages of using reactions with higher atom economies?

  They use resources slower, produce less waste, and are more profitable
• What is percentage yield and its formula?

  • Percentage yield = (mass of product actually made / maximum theoretical mass of product) × 100
  • It compares actual product yield with theoretical yield.
• Why are yields always lower than 100%?

  Yields are lower due to incomplete reactions, side reactions, and product loss during separation