C4: Chemical Calculations


C4.1: Relative Masses and Moles

  • the masses of atoms are compared by measuring them relative to atoms of carbon-12. 
  • the relative formula mass of a compound can be worked out by adding up the relative atomic masses of the elements in it, in the ration shown by the formula.
  • one mole of any substance is the relative formula mass in grams
  • number of moles = mass in grams/ relative formula mass
  • avogadro's constant is 6.02 x 10^23
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C4.2: Equations and Calculations

  • balanced symbol equations tell you the number of moles of substances involved in a chemical reaction
  • you can use balanced symbol equations to calculate the masses of reactants and products in a chemical reaction
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C4.3: From Masses to Balanced Equations

  • balanced symbol equations can be deduced by the masses (and the ratio of the numbers of moles) of substances involved in a chemical reaction
  • the reactant that gets used up first is the limiting factor
  • this is the reactant that is NOT in excess
  • amounts of product formed in a chemical reaction are determined by the limiting reactant. 
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C4.4: Expressing Concentrations

  • concentration is measures in g/dm^3
  • concentration = amount of solute (g)  / volume of solution (dm^3)
  • dm^3 = 1000cm^3
  • to calculate the mass of a solute in a certain volume with a known concentration:
    • calculate the mass in grams for 1dm^3 of solution
    • calulate the mass in grams of solute im 1cm^3 (divide by 1000)
    • calculate the mass in grams of solute in the given volume of solution
  • a more concentrated solution has more solute in the same volume of solution than a less concentrated solution
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