Fundamental ideas 2

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Relative masses and moles

  • The relative atomic mass of an element (Ar) compares the mass of atoms of the element with the 12C isotope. It is an average value for the isotopes of the element.
  • The relative formula mass (Mr) of a compound is the sum of the relative atomic masses of the atoms in the numbers shown in the formula
  • The relative formula mass of a substance, in grams, is known as one mole of that substance
  • A mole of any substance always contains the same number of atoms, molecules or ions - 6.02x1023 (Avogadras number)


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Percentages and formulae

The percentage by mass of an element in a compound can be calculated from the relative atomic mass of the element in the formula and the relative formula mass of the compound


The empirical formula of a compound can be calculated from the masses or percentages of the element in a compound

  • List all the elements in the compound
  • Underneath, write the experimental masses or percentages
  • Divide each mass or percentage by the Ar of that element
  • Find the simplest whole number ratio of the elements
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Chemical equations

  • Chemical equations show the reactants and the products
  • (g) - gas
  • (l) - liquid
  • (s) - solid
  • (aq) - solution
  • No atmons are lost or made during a chemical reaction so the mass of the products equals the mass of the reactants
  • `The masses of reactants can be calculated from balanced symbol equations

eg   H2 + 02 -->  H20

     2H2 + 02 --> 2H20

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Equations and calculations

Using balanced equations to work out reacting masses

             eg Hydrogen + Chlorine --> Hydrochloric acid

                       H2       +      Cl2     -->           2HCl

Ar/Mr                2                 71                     36.5

mol                   1                  1                         2

mass                 2                 71                       73

     eg sodium hydroxide + chlorine -->  bleach +  salt  + water    

                 2NAoH           +    Cl2     ---> NaOCl  + NaCl + H2O  

Ar/Mr            40                      71                      

mass          100g                    88.75g

mol               2.5                     1.25

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`Percentage yield

Percentage yield compares what you calculate you should get (predicted yield) with what you get in practice (actual yield) 


Yields are always less than 100% 

  • The reaction is reversible - the products of the reaction can react to produce the original reactants


  • Some of the product may be lost when it is separated from the reaction mixture
  • Some of the reactants may react in different ways different from the expected reaction
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