Relative formula mass

• What is the topic of today's video?

  Relative formula mass calculations
• How can we find the mass number of an element?

  By looking in the top left corner of its nuclear symbol
• What does the mass number represent?

  Number of protons plus neutrons
• What is relative atomic mass denoted by?

  ar
• What is the relative atomic mass of chlorine as an element?

  35.5
• How do we find the average mass of an entire compound?

  By calculating the relative formula mass (mr)
• What is the molecular formula of magnesium chloride?

  MgCl₂
• How do you calculate the relative formula mass of magnesium chloride?

  Add 24 and 35.5 times 2
• What is the relative formula mass of magnesium chloride?

  95
• What is the molecular formula of sulfuric acid?

  H₂SO₄
• How do you calculate the relative formula mass of sulfuric acid?

  Calculate 2 times 1 plus 32 plus 4 times 16
• What is the relative formula mass of sulfuric acid?

  98
• How do you calculate the percentage mass of sulfur in sulfuric acid?

  Use ar of sulfur times number of atoms divided by mr
• What is the percentage mass of sulfur in sulfuric acid?

  32.7%
• What is the formula for calcium hydroxide?

  Ca(OH)₂
• How do you calculate the percentage mass of oxygen in calcium hydroxide?

  Use 16 times 2 divided by mr
• What is the percentage mass of oxygen in calcium hydroxide?

  43.2%
• What is the process to find the relative formula mass of a compound?

  • Identify the molecular formula
  • Find the relative atomic masses of each element
  • Sum the relative atomic masses of all atoms in the formula
• What is the formula to calculate percentage mass of an element in a compound?

  • Percentage mass = (ar of element × number of atoms) / mr of compound × 100
• What are the steps to calculate the relative formula mass of magnesium chloride?

  1. Identify the formula: MgCl₂
  2. Find ar of magnesium: 24
  3. Find ar of chlorine: 35.5
  4. Calculate: 24 + (35.5 × 2) = 95
• What are the steps to calculate the relative formula mass of sulfuric acid?

  1. Identify the formula: H₂SO₄
  2. Find ar of hydrogen: 1
  3. Find ar of sulfur: 32
  4. Find ar of oxygen: 16
  5. Calculate: (2 × 1) + 32 + (4 × 16) = 98
• What are the steps to calculate the percentage mass of sulfur in sulfuric acid?

  1. Find ar of sulfur: 32
  2. Find mr of sulfuric acid: 98
  3. Calculate: (32 × 1) / 98 × 100 = 32.7%
• What are the steps to calculate the percentage mass of oxygen in calcium hydroxide?

  1. Identify the formula: Ca(OH)₂
  2. Find ar of calcium: 40
  3. Find ar of oxygen: 16
  4. Find ar of hydrogen: 1
  5. Calculate: (16 × 2) / 74 × 100 = 43.2%