Chemical calculations

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  • Created on: 12-04-15 10:48
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  • Chemical calculations
    • Atomic and molecular masses
      • Masses of atoms
        • Carbon-12 is the standard of reference.
        • Relative atomic mass- average mass of one atom of the element relative to 1/12 the mass of one atom of carbon-12
          • Use relative isotopic mass when referring to isotopes.
      • Masses of molecules
        • Relative molecular mass- the average mass of a molecule relative to one-twelfth the mass of an atom of carbon-12
      • Amount of substance
        • One mole- the amount of any substance that contains the same number of particles as there are atoms in exactly 12g of carbon-12
          • Mass per mole is the molar mass
          • Avogadros constant is 6.02*10^23. (Number of atoms in 12g carbon-12)
    • Mass spectrometer
      • Ionisation-  gaseous sample bombarded with high energy electrons, forms positive ions
        • Acceleration- electric fields acceleration +ve ions to high speed
          • Deflection- magnetic field deflects ions according to mass/charge ratio.
            • Detection- ions with correct mass/charge ratio pass through slit and are detected. Signal amplified and recorded.
              • High vacuum to prevent collision with air molecules
      • Relative atomic mass is weighted average of the masses of all the atoms in the isotopic mixture.
      • Mass spectrum of chlorine
        • Because Cl exists as a molecule, some become molecular ions and others fragment.
          • Ratio of atomic ions- 35Cl+ : 37Cl+ is 3:1
          • Ratio of molecular ions: mass 70:72:74 is 9:6:1
    • Empirical and molecular formulae
      • Empirical formula- simplest formula showing simplest whole number ratio of elements.
      • Molecular formula- shows actual number of atoms of each element
    • Moles, mass and concentration
      • n = m / M
        • Where n=moles, m=mass, M=molar mass
      • n=c x v
        • Where n=moles, c=concentration, v=volume.
          • Concentration tells you how much solute is dissolved in the solvent.
        • Remember to convert volumes to dm^3
    • Equations for solids and gas reactions
      • Equations tell us what substances react together and the amounts
        • Number of moles given in balanced equation are called stoichiometric amounts
      • For solids- moles calculated from masses
        • For gases, moles calculated from volumes using molar gas volume.
    • Atom economy and percentage yield
      • In a reaction, the compounds formed, other than the product needed, are waste
        • Atom economy- mass of required product / total mass of reactants * 100%
        • Percentage yield- mass of product obtained / maximum theoretical mass * 100%
          • Give indication of the efficiency of a reaction
            • Atom economy- mass of required product / total mass of reactants * 100%
            • The higher the atom economy, the more efficient the process


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