Relative Formula mass

• What is the topic of the study material?

  Quantitative Chemistry
• What does M_r stand for?


  Relative Formula Mass
• How do you calculate the percentage mass of an element in a compound?

  Percentage mass = $\frac{A_r \times \text{number of atoms}}{M_r} \times 100$
• What is the relative formula mass of MgCl₂?


  It is the sum of the atomic masses of magnesium and chlorine in the compound.
• What is the atomic mass (A_r) of sodium?


  23
• What is the atomic mass (A_r) of carbon?


  12
• What is the atomic mass (A_r) of oxygen?


  16
• What is the relative formula mass (M_r) of sodium carbonate, Na₂CO₃?


  M_r = (2 × 23) + 12 + (3 × 16) = 106
• How do you find the percentage mass of sodium in sodium carbonate?


  Percentage mass of sodium = $\frac{23 \times 2}{106} \times 100 =$$43\%$
• What is the percentage mass of sodium in sodium carbonate?


  43%
• If a mixture contains 20% iron ions by mass, how much iron is in 50 g of the mixture?


  10 g of iron
• How do you calculate the mass of iron in a mixture containing 20% iron by mass?


  Mass of iron = $50 \times \frac{20}{100} =$$10 \text{ g}$
• What is the percentage mass of iron in iron chloride (FeCl₃)?


  Percentage mass of iron = $\frac{56}{162.5} \times 100 \approx 34.4\%$
• How much iron chloride is needed to provide 10 g of iron?


  You need approximately 29 g of iron chloride.
• How do you calculate the mass of iron chloride that contains 10 g of iron?


  Mass of iron chloride = $\frac{10}{34.4} \times 100 \approx 29 \text{ g}$
• What are the steps to calculate the relative formula mass (M_r) of a compound?


  1. Identify the elements in the compound.
  2. Find the atomic masses (A_r) of each element.
  3. Multiply the atomic mass by the number of atoms of each element.
  4. Sum all the values to get M_r.
• What are the steps to calculate the percentage composition by mass of an element in a compound?

  1. Find the atomic mass (A_r) of the element.
  2. Calculate the relative formula mass (M_r) of the compound.
  3. Use the formula:

  $Percentage mass =$$\frac{A_r \times \text{number of atoms}}{M_r} \times 100$
• What is the atomic mass (A_r) of chlorine?


  35.5
• What is the relative formula mass (M_r) of iron chloride (FeCl₃)?


  162.5
• What is the significance of practicing calculations in quantitative chemistry?


  It helps to understand and apply concepts effectively.
• What is the relative formula mass (M_r) of water (H₂O)?


  M_r = (2 × 1) + 16 = 18
• What is the relative formula mass (M_r) of lithium hydroxide (LiOH)?


  M_r = 7 + 1 + 16 = 24
• What is the relative formula mass (M_r) of sulfuric acid (H₂SO₄)?


  M_r = (2 × 1) + 32 + (4 × 16) = 98
• What are the example calculations provided in the study material?

  1. Relative formula mass of MgCl₂
  2. Percentage mass of sodium in Na₂CO₃
  3. Mass of iron chloride needed for iron ions in a mixture
• What is the importance of calculating relative formula mass and percentage mass in chemistry?


  • Essential for understanding compound composition
  • Useful in stoichiometry for reactions
  • Helps in determining the amounts needed in reactions