CIE A Level Chemistry复习笔记1.2.5 Empirical & Molecular Formulae

Empirical & Molecular Formulae

  • The molecular formula is the formula that shows the number and type of each atom in a molecule
    • Eg. the molecular formula of ethanoic acid is C2H4O2
  • The empirical formula is the simplest whole number ratio of the elements present in one molecule or formula unit of the compound
    • Eg. the empirical formula of ethanoic acid is CH2O
  • Organic molecules often have different empirical and molecular formulae
  • Simple inorganic molecules however have often similar empirical and molecular formulae
  • Ionic compounds always have similar empirical and molecular formulae

Empirical & Molecular Formulae Calculations

Empirical formula

  • Empirical formula is the simplest whole number ratio of the elements present in one molecule or formula unit of the compound
  • It is calculated from knowledge of the ratio of masses of each element in the compound
  • The empirical formula can be found by determining the mass of each element present in a sample of the compound
  • It can also be deduced from data that give the percentage compositions by mass of the elements in a compound

Worked Example: Empirical formula from mass

1.2-Atoms-Molecules-Stoichiometry-Worked-Example-Empirical-formula-from-mass_1

Worked Example 2: Empirical formula from %

1.2-Atoms-Molecules-Stoichiometry-Worked-Example-Empirical-formula-from-percentage_1

Molecular formula

  • The molecular formula gives the exact numbers of atoms of each element present in the formula of the compound
  • The molecular formula can be found by dividing the relative formula mass of the molecular formula by the relative formula mass of the empirical formula
  • Multiply the number of each element present in the empirical formula by this number to find the molecular formula

Worked example: Calculating molecular formula

1.2-Atoms-Molecules-Stoichiometry-Worked-Example-Calculating-molecular-formula

Answer

  • Step 1: Calculate relative formula mass of empirical formula

Relative formula mass = (C x 4) + (H x 10) + (S x 1)

Relative formula mass = (12 x 4) + (1 x 10) + (32 x 1)

Relative formula mass = 90

  • Step 2: Divide relative formula mass of X by relative formula mass of empirical formula

Ratio between Mr of X and the Mr of the empirical formula = 180/90

Ratio between Mr of X and the Mr of the empirical formula = 2

  • Step 3: Multiply each number of elements by 2

(C4 x 2) + (H10 x 2) + (S1 x 2)     =    (C8) + (H20) + (S2)

Molecular Formula of X is C8H20S2

 

 

 

 

 

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