Chemistry 1.3 Chemical calculations

• Relative atomic mass


  The average mass of one atom of an element compared to 1/12th the mass of an atom of carbon-12
• Relative formula mass


  The weighted average of the masses of the formula units compared to 1/12th the mass of an atom of carbon-12
• Relative molecular mass


  The weighted average of the masses of the molecules compared to 1/12th the mass of an atom of carbon-12
• Relative isotopic mass


  The mass of one atom of an isotope compared to 1/12th the mass of an atom of carbon-12
• Mass spectrometry


  A type of analytical technique which measures the mass to charge ratio of ions and how abundant each ion is, so it can be used to calculate the relative atomic mass
• TOF mass spectrometry


  1. Ionisation

  2. Acceleration

  3. Ion drift

  4. Detection

  5. Analysis
• [(79.2 x 17) + (80.1 x 83)] ÷ 100 = 79.95 g mol-1
• Molecular ion peak (M+)


  The peak with the highest m/z value, caused by the whole molecule, therefore m/z value = molecular mass
• Possible combinations of the Cl2+ are 70, 72, 74 in the ratio 9:6:1. There are also peaks at m/z 35 and 37 due to fragmentation.
• Empirical formula


  The simplest whole number ratio of atoms of each element in a compound
• Molecular formula


  The actual number of atoms of each element present in a compound
• The empirical formula for Fe2O4 is FeO2
• Calculating molecular formula from empirical formula and relative molecular mass

  Calculate empirical mass

  2. Compare molecular mass with empirical mass

  3. Molecular formula is multiple of empirical formula
• Avogadro constant


  The number of particles in one mole of a substance, 6.02 x 10^23 particles
• Equation linking Avogadro's constant to moles

  Number of particles = Moles x Avogadro's constant
• There are 8 mol of Fe in the sample
• Molar mass


  The mass per mole of a substance, measured in g mol-1
• Equation linking moles to mass

  Moles = Mass (g) x Relative atomic mass
• Concentration


  The amount of moles per unit volume
• Calculating concentration in g/dm3

  Concentration (g/dm3) = Mass (g) / Volume (dm3)
• Calculating concentration in mol/dm3

  Concentration (mol/dm3) = Moles / Volume (dm3)
• Molar volume of any gas at room temperature and pressure (RTP)

  24 dm3
• RTP


  Room temperature and pressure: 20oC, 1 atmosphere
• Equation linking molar volume at RTP to moles


  Volume of gas at RTP (dm3) = moles x 24
• Increasing temperature at a given pressure

  Reduces the molar volume
• Increasing pressure at a given temperature

  Reduces the maximum molar volume, but up to this maximum, increasing pressure will increase the molar volume