EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:1.5.8 Empirical & Molecular Formulae

EDEXCEL IGCSE CHEMISTRY: DOUBLE SCIENCE 复习笔记:1.5.8 Empirical & Molecular Formulae

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 the atoms of each element present in one molecule or formula unit of the compound
    • E.g. the empirical formula of ethanoic acid is CH2O

     

  • Organic molecules often have different empirical and molecular formulae
  • The formula of an ionic compound is always an empirical formula

 

 

Calculate Empirical & Molecular Formulae

Empirical formula: gives the simplest whole number ratio of atoms of each element in the compound

  • It is calculated from knowledge of the ratio of masses of each element in the compound

 

Example:A compound that contains 10 g of hydrogen and 80 g of oxygen has an empirical formula of H2O. This can be shown by the following calculations:Amount of hydrogen atoms = mass in grams ÷ Ar of hydrogen = (10 ÷ 1) = 10 molesAmount of oxygen atoms = mass in grams ÷ Ar of oxygen = (80 ÷ 16) = 5 moles 

The Ratio of Moles of Hydrogen Atoms to Moles of Oxygen Atoms

 

4.2-The-Ratio-of-Moles-of-Hydrogen-Atoms-to-Moles-of-Oxygen-Atoms-table Since equal numbers of moles of atoms contain the same number of atoms, the ratio of hydrogen atoms to oxygen atoms is 2:1Hence the empirical formula is H2OMolecular formula: gives the exact numbers of atoms of each element present in the formula of the compound

  • Divide 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

 

Relationship between Empirical and Molecular Formula

 

4.2-Relationship-between-Empirical-_-Molecular-Formula-table 

Worked Example

The empirical formula of X is C4H10S1 and the relative formula mass of X is 180. What is the molecular formula of X?Relative atomic masses:       carbon : 12      hydrogen : 1      sulfur : 32

 

Step 1 - Calculate the relative formula mass of the empirical formula

 

(C x 4) + (H x 10) + (S x 1)    =   (12 x 4) + (1 x 10) + (32 x 1)   =   90

 

Step 2 - Divide the relative formula mass of X by the mass of the empirical formula

 

180 / 90 = 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 = C8H20S2

转载自savemyexams

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