Relative formula mass(RFM)

• What is the topic of the study material provided?

  Quantitative Chemistry
• What does relative formula mass represent in a compound?

  It is the sum of the relative atomic masses of all the atoms in the molecular formula.
• How do you find the relative formula mass of a compound like MgCl₂?

  Add the relative atomic masses of magnesium and chlorine from the periodic table.
• What is the relative atomic mass of magnesium (Mg)?

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

  35.5
• How do you calculate the percentage mass of an element in a compound?

  Use the formula: Percentage mass of an element = (number of atoms of that element × M of the element / M of the compound) × 100.
• What is the percentage mass of sodium in sodium carbonate (Na₂CO₃)?

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

  Calculate 20% of 50 g, which is 10 g of iron.
• How do you calculate the percentage mass of iron in iron chloride (FeCl₃)?

  Use the formula: Percentage mass of iron = (number of atoms of iron × M of iron / M of FeCl₃) × 100.
• What is the relative atomic mass of iron (Fe)?

  56
• What is the relative atomic mass of carbon (C)?

  12
• How much iron chloride is needed to provide 10 g of iron if iron chloride contains 44.09% iron by mass?

  You need 23 g of iron chloride.
• What are the steps to calculate the relative formula mass of potassium hydroxide (KOH)?


  1. Find the relative atomic mass of potassium (K), oxygen (O), and hydrogen (H).
  2. Add the relative atomic masses:

  • K = 39
  • O = 16
  • H = 1

  1. Calculate:

  • M(KOH) = 39 + 16 + 1 = 56
• How do you calculate the percentage composition by mass of potassium in potassium hydroxide (KOH)?

  1. Find the relative atomic mass of potassium (K) = 39.
  2. Calculate the total mass of KOH = 56.
  3. Use the formula:

  • Percentage mass of potassium = (39 / 56) × 100 ≈ 69.64%
• What are the steps to calculate the percentage composition by mass of sulfuric acid (H₂SO₄)?

  1. Find the relative atomic masses:

  • H = 1, S = 32, O = 16

  1. Calculate the total mass of H₂SO₄ = (2 × 1) + 32 + (4 × 16) = 98.
  2. Calculate the percentage mass of each element:

  • H: (2 / 98) × 100
  • S: (32 / 98) × 100
  • O: (64 / 98) × 100