Masses and empirical formula

• What is an empirical formula?

  It is the simplest whole number ratio of atoms or ions of each element in a substance.
• What is the empirical formula of silicon dioxide?


  SiO₂
• What does the empirical formula of SiO₂ indicate about the ratio of silicon to oxygen atoms?

  The ratio of silicon atoms to oxygen atoms is 1:2.
• What is a molecular formula?

  It represents the actual number of atoms of each element in one molecule.
• What is the molecular formula of ethene?

  C₂H₄
• What is the empirical formula of ethene?

  CH₂
• When is the empirical formula the same as the molecular formula?

  When the formula cannot be simplified any further, like in the case of water (H₂O).
• What is the formula for a simple molecule?

  It is its molecular formula.
• What is the relative formula mass (Mᵣ) of a substance?

  It is the sum of the relative atomic masses (Aᵣ) of all the atoms or ions in its formula.
• How do you calculate the Mᵣ of carbon dioxide (CO₂)?

  Mᵣ = Aᵣ(C) + (2 × Aᵣ(O)) = 12 + (2 × 16) = 44
• What is the Mᵣ of calcium nitrate (Ca(NO₃)₂)?

  Mᵣ = Aᵣ(Ca) + 2(Aᵣ(N) + (3 × Aᵣ(O))) = 40 + 2(14 + (3 × 16)) = 164
• How do you find the empirical formula from the masses of elements used to make it?

  1. Divide the mass of each element by its relative atomic mass.
  2. Divide the answers by the smallest number to find the simplest ratio.
  3. Write the empirical formula based on the simplest ratio.
• What is the empirical formula of calcium chloride if 10.0g of calcium reacts with 17.8g of chlorine?

  CaCl₂
• What is the empirical formula of lithium oxide if 1.4g of lithium and 1.6g of oxygen are used?

  Li₂O
• What is the empirical formula of magnesium carbonate if 1.2g of magnesium, 0.6g of carbon, and 2.4g of oxygen are used?

  MgCO₃
• What is the empirical formula of butane if 1.44g of carbon and 0.3g of hydrogen are used?

  C₄H₁₀
• How do you determine the molecular formula from the empirical formula and Mᵣ?

  Divide Mᵣ by the empirical formula mass and multiply the empirical formula by that number.
• What is the molecular formula of glucose if its empirical formula is CH₂O and Mᵣ is 180?

  C₆H₁₂O₆
• What is the empirical formula of magnesium oxide if 0.576g of magnesium produces 0.960g of magnesium oxide?

  MgO
• What is the empirical formula of iron bromide if 5.6g of iron reacts with 24.0g of bromine?

  FeBr₃
• What is the process to calculate the empirical formula from the mass of elements used in a reaction?

  1. Measure the mass of each element.
  2. Divide each mass by its relative atomic mass.
  3. Find the simplest ratio by dividing by the smallest value.
  4. Write the empirical formula based on the ratio.
• What is the difference between empirical and molecular formulas?

  • Empirical formula: simplest whole number ratio of elements.
  • Molecular formula: actual number of atoms in a molecule.
• What is the significance of relative atomic masses in calculating Mᵣ?

  • They are used to sum the masses of all atoms in a formula.
  • Found in the periodic table.