C3

• What is the relative formula mass of a compound represented as?

  M_r
• How is the relative formula mass of a compound calculated?

  By adding the relative atomic masses of all the atoms in the molecular formula.
• What is the relative formula mass of MgCl₂?

  95
• What is the relative atomic mass of magnesium (Mg)?


  24
• What is the relative atomic mass of chlorine (Cl)?

  35.5
• Why do you multiply the relative atomic mass of chlorine by 2 in MgCl₂?

  Because there are two chlorine atoms in the compound.
• What is the formula to calculate the percentage mass of an element in a compound?

  Percentage mass = (A_r × number of atoms) / M_r × 100
• What is the percentage mass of sodium in sodium carbonate (Na₂CO₃)?

  43%
• How do you find the percentage mass of iron in iron chloride (FeCl₃)?

  By calculating the A_r of iron and chlorine and using the percentage mass formula.
• If a mixture contains 20% iron ions by mass, how much iron is in 50 g of the mixture?

  10 g of iron.
• What does it mean when mass is conserved in a chemical reaction?

  No atoms are created or destroyed, so the total mass remains the same.
• What is the balanced equation for the reaction of lithium and fluorine?

  2Li + F₂ → 2LiF
• How can you show that mass is conserved in the reaction 2Li + F₂ → 2LiF?

  By adding the relative formula masses on both sides and confirming they are equal.
• What happens to the mass of a reaction vessel if a gas escapes during a reaction?

  The mass of the reaction vessel decreases.
• How much iron chloride is needed to provide 10 g of iron if it contains 44.09% iron by mass?

  23 g of iron chloride.
• Why does the mass of a reaction vessel increase when a metal reacts with oxygen in an unsealed container?

  Because the gas (oxygen) is now contained in the vessel after the reaction.
• What is the Avogadro constant?

  1. 023 x 10²³ particles.
• What is the definition of a mole in chemistry?

  A mole is the amount of a substance containing Avogadro's number of particles.
• What does it mean when we say mass is conserved during a chemical reaction?

  It means that the total mass of reactants equals the total mass of products.
• What can cause a perceived change in mass during a reaction?

  The escape of gas or the absorption of gas into the reaction vessel.
• What are the steps to calculate the relative formula mass of a compound?

  1. Look up the relative atomic masses of all elements in the compound.
  2. Add the relative atomic masses together.
  3. Consider the number of atoms of each element in the compound.
• How do you calculate the percentage composition by mass of an element in a compound?

  • Use the formula:

  Percentage mass = (A_r × number of atoms) / M_r × 100

  • Substitute the values for A_r and M_r.
• What are the implications of the conservation of mass in chemical reactions?

  • No atoms are created or destroyed.
  • The total mass of reactants equals the total mass of products.
  • Changes in mass may indicate gas involvement.
• What are the two explanations for mass changes in an unsealed reaction vessel?

  1. Mass increases if a gas from the air reacts and is contained.
  2. Mass decreases if a gas product escapes into the air.
• What is the significance of the Avogadro constant in chemistry?

  • It defines the number of particles in one mole of a substance.
  • It is approximately 6.023 x 10²³.
• How can you use moles to calculate masses in reactions?

  • Use balanced equations to relate moles of reactants and products.
  • Convert moles to mass using molar mass.
• What happens to gas if the reaction vessel is not enclosed?

  The gas can escape from the reaction vessel as it’s formed.
• Why does the total mass of the contents in an unsealed reaction vessel decrease?

  Because the gas escapes and is no longer accounted for in the mass.
• What occurs when a metal carbonate thermally decomposes in an unsealed vessel?

  The mass of the reaction vessel will decrease if it isn’t sealed.
• What is the general reaction for the thermal decomposition of a metal carbonate?

  Metal carbonate → metal oxide + carbon dioxide.
• How does a gas behave in a container according to the particle model?

  A gas will expand to fill any container it’s in.
• What do the big numbers in front of chemical formulae in balanced equations represent?

  They represent the number of moles of each substance involved in the reaction.
• In the reaction Mg + 2HCl → MgCl₂ + H₂, how many moles of hydrochloric acid react with one mole of magnesium?

  Two moles of hydrochloric acid react with one mole of magnesium.
• What are the steps to balance a chemical equation using reacting masses?

  1. Divide the mass of each substance by its relative formula mass to find moles.
  2. Divide the number of moles by the smallest number of moles.
  3. Multiply to get whole numbers if necessary.
  4. Write the balanced symbol equation.
• How do you calculate the relative formula mass of zinc oxide (ZnO)?

  A_r(Zn) + A_r(O) = 65 + 16 = 81.
• How do you find the number of moles of a substance from its mass and relative formula mass?

  Divide the mass of the substance by its relative formula mass.
• What is the ratio of moles of ZnO to C in the reaction ZnO + C → CO₂ + Zn?

  The ratio is 2:1 for ZnO to C.
• What is the balanced equation for the reaction of zinc oxide and carbon?

  2ZnO + C → CO₂ + 2Zn.
• What is a limiting reactant in a chemical reaction?

  A limiting reactant is the substance that is completely consumed first, stopping the reaction.
• How do you calculate the mass of aluminium oxide formed from burning aluminium?

  By using the balanced equation and the moles of aluminium burned.