C3-Quantitative Chemistry

• What is the formula for Relative Formula Mass (Mr)?

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

  Percentage mass = (number of atoms of that element / Mr of the compound) x 100
• What is the formula for concentration?

  Concentration = amount of substance dissolved in a certain volume of solution
• How is concentration expressed in terms of mass and volume?

  Concentration = mass of solute (g) / volume of solution (dm³)
• What can be used to find the uncertainty in the mean value of repeated measurements?

  The range of the results can be used to find the uncertainty
• What is a balanced equation?

  A balanced equation has the same number of atoms of each element on both sides
• What is the balanced equation for the reaction of magnesium with oxygen?

  2Mg + O₂ → 2MgO
• 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 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 an example of a reaction that produces a gas and can lead to a decrease in mass?

  The thermal decomposition of calcium carbonate produces CO₂ gas
• What happens to the mass in an unsealed 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 is an example of a reaction that leads to an increase in mass due to a gas from the air being a reactant?

  The reaction of magnesium with O₂ gas produces a solid
• What are the key principles of conservation of mass in chemical reactions?

  • No atoms are created or destroyed
  • Total mass of reactants equals total mass of products
  • Changes in mass can occur due to gas escape or gas intake
• What are the implications of mass changes in unsealed reaction vessels?

  • Decrease in mass: gas escapes (e.g., CO₂ from calcium carbonate)
  • Increase in mass: gas enters from air (e.g., O₂ in magnesium reaction)