C2.3 Atomic structure, Analysis and Quantitative Chemistry


C2.3.1 Atomic Structure

Atoms can be represented in this way:


  • Mass number = total number of protons AND neutrons in an atom
  • Atomic niumber = number of protons in an atom
  • Number of neutrons = mass number - atomic number

Isotopes of an element have the same atomic number but different mass numbers. They react in the same way as the element as they have the same electronic structure.

Proton = relative mass 1
Neutron = relative mass 1
El;ectron = relative mass very small

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C2.3.1 Relative Formula Mass

The Relative Formula Mass

  • All the relative atomic masses of all the atoms shown in its formula added together
  • Has the symbol Mr
  • Has no units


  • The Mr of a substance in grams = 1 mole of the substance
  • e.g Oxygen = O2
    • Mr of O2 = 2(Ar of O) = 32
    • 32g of O2 = 1 mole of oxygen

Relative Atomic Mass

  • Ar
  • Average value for the isotopes of the element
  • Compares mass of atoms of the element with the 12C isotope
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C2.3.3 Chemical Calculations

A percentage composition is a measure of how much of a particular element is present in a compound

You need to know:

  • Number of atoms of the element in the compound
    • Shown by the chemical formula
  • Relative atomic mass (Ar) of the compound
    • Given in the question
  • Relative formula mass (Mr) of the compound
    • Calculated from the formula and Ar Values

How to do it:

  • Calculate Mr first
  • Multiply number of atoms by Ar 
  • Then divide this by Mr
  • Multiply by 100
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C2.3.3 More Chemical Calculations

Empirical Formula - The simplest whole number formula for a compound

  • List each chemical symbol
  • Calculate each mass/Ar or percentage/Ar
  • Divide each number from step 2 by the smallest number to find the simplest whole numbers

Mass Calculations

Mass of product = mass of reactant / total Mr of reactant x total Mr of product

Percentage yield 

Percentage yield is the actual yield is the actual yield compared to the teoretical yield: 0% means no expected product was obtained, 100% means that all of the expected product was obtained

Percentage yield = actual yield / theoretical yield x 100

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C2.3.3 Reactions

The amount of a product obtained in a chemical process is its yield.No atoms are lost or gained in a chemical reaction.

The Theoretical Yield

  • Maximum mass of products for a given mass
  • Calculated by using the balanced symbol equation for the reaction and the relative masses of the substances involved

The Actual Yield

  • Amount obtained when reaction is carried out and any purification steps have been done
  • Usually less than theoretical yield:
    • Reaction may be reversible so may not go to completion
    • Some product may be lost when separated from the reaction mixture
    • Some reactants may react in ways different from the expected reaction
      • May produce by-products

Reversible reactants are where products can react to make the original reactants
Equations represent reversible reactions with a split arrow pointing in both directions:(http://image.tutorvista.com/content/feed/u145/arrow.JPG)

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