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AQA A Level Chemistry復(fù)習(xí)筆記1.2.2 Empirical & Molecular Formula

Category: A-level課程, 教材筆記, 福利干貨 Date: 2022年6月30日 上午10:13

Empirical & Molecular Formulae

 

  • The?molecular?formula?is the formula that shows the?number?and?type?of each atom in a molecule
    • E.g. the molecular formula of ethanoic acid is C2H4O2

     

  • The?empirical?formula?is the simplest whole number ratio of atoms of each element present in one molecule or formula unit of a compound
    • E.g. the empirical formula of ethanoic acid is CH2O

     

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 gives the?percentage compositions by mass?of the elements in a compound

Worked Example

Empirical formula from mass

Determine the empirical formula of a compound that contains 10 g of hydrogen and 80 g of oxygen.

WE-Atoms-Molecules-Stoichiometry-Empirical-formula-from-mass_2

 

  • The above example shows how to calculate empirical formula from the mass of each element present in the compound
  • The example below shows how to calculate the empirical formula from percentage composition

Worked Example

Empirical formula from %

Determine the empirical formula of a compound that contains 85.7% carbon and 14.3% hydrogen.

WE-Atoms-Molecules-Stoichiometry-Empirical-formula-from-percentage_2

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

The empirical formula of X is C4H10S and the relative molecular mass of X is 180
What is the molecular formula of X?

(Ar?data: C = 12, H = 1, S = 32)

Answer

Step 1:?Calculate relative mass of the empirical formula

    • Relative empirical mass = (C x 4) + (H x 10) + (S x 1)
    • Relative empirical mass = (12 x 4) + (1 x 10) + (32 x 1)
    • Relative empirical mass = 90

     

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

    • 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) + (S?x 2) ????= ???(C8) + (H20) + (S2)
    • Molecular Formula of?X?is C8H20S2

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