Empirical Formula (OCR Gateway GCSE Chemistry: Combined Science)

• Empirical formula is the simplest whole number ratio of atoms of each element in a molecule 
• The molecular formula tells you the actual number of atoms of each element in a molecule
• From the molecular formula, we can deduce the empirical formula 
• The molecular formula will either be given to you or can be worked out from a picture 
• Sometimes, the empirical formula and molecular formula are the same e.g. CO₂

Answer:

• • Divide the number of atoms for each element by a common factor 
  • In this case, both can be divided by 2 
  • The empirical formula is C₃H₇

Answer: 

• • Count the number of atoms of each element: C= 4 H= 10
  • Divide both numbers by a common factor, in this case 2 
  • The empirical formula is C₂H₅

Empirical Formula of Ionic Compounds

• If it is a dot-and-cross diagram, then just count the number of atoms of each element
• This is then equal to the empirical formula of the compound
• If it is a 3D lattice structure, identify the ions in the lattice
• Write them down and balance the charges so that the overall charge is zero 
• The formula of the compound is the empirical formula 

Answer:      

• • Identify the ions in the compound, in this case Na+ and Cl- 
  • Balance the charges so that the overall charge is zero
  • Only one chloride ion is needed to balance out the single positive charge of the sodium ion
  • The empirical formula is NaCl 

• The percentage by mass of an element in a compound can be calculated using the following equation:

Answer