Essential Measurements

• Practical skills: Essential Measurements

  • Measuring mass
  • Measuring volume of liquid
  • Measuring volume of gas
  • Measuring temperature
  • Measuring pH
  • Measuring colour (and thus concentration)
• Measuring mass


  • Used in required practicals: 1, 2, 3, 7 and 10
  • The before and after weighing method (weighing-by-difference) is more accurate than weighing on a scale directly
  • The weight of the material is the difference between the two weights of the weighing boat before and after you transfer the material from the weighing boat into the beaker
  • If the weighing boat weights more than 0g after transfer there is solid left on the weighing boat
  • In the example of making a standard solution you should wash the weighing boat with distilled water
• Measuring volume of liquid

  • Used in required practicals: 1, 2, 3, 4, 5, 6, 7, 8, 9, and 11
  • For identification tests, error in volume of liquid added is not significant, so using a dropping pipette or beaker is sufficient
  • For analysis or titrations, a dropping pipette cannot be used because volume in each drop varies, so a volumetric pipette should be used
  • Volumetric pipettes can accurately measure a volume of 25 cm3 to a very high precision (uncertainty is very low at ±0.03cm3)
  • Hold burettes or volumetric pipettes vertically and read the meniscus at eye level to avoid parallax error
  • In a titration, you can reduce the percentage error in the volume of the titre by increasing the volume of the titre
• Measuring volume of gas

  • Used in required practicals: 3 and 7
  • A gas syringe or trough should be used to measure a volume of gas
  • The syringe is more easily operated but has a greater uncertainty than the trough
  • The measuring cylinder must be completely full of water at before the beginning of the practical, and the meniscus will fall on the volume of gas produced that collects at the top of the cylinder, giving a more precise volume than what is read in the syringe
• Measuring temperature


  • Used in required practicals: 2, 3, and 5
  • In an enthalpy change practical, the temperature change needs to be accurately measured by minimising heat loss or heat gain from the surroundings using a polystyrene cup and lid, and ensuring the thermometer is in the reaction mixture
  • In required practical 5, distillation, the temperature must be measured at intervals to check that it is close to the liquid's boiling point, with the thermometer at the top of the distillation column and parallel to the start of the condenser tube

  
• Measuring pH


  • Used in required practical: 9
  • To reduce uncertainty, you must stir the solution and move the pH probe before reading to ensure the H+ concentration is homogenous
  • You should construct a pH calibration curve before the acid-base titration to account for systematic error in the pH meter due to decalibration over time

  
• Measuring colour (and thus concentration)

  1. Add an appropriate ligand to produce a complex ion with colour, ensuring the ligand is added in excess

  2. Make up solutions of known concentration

  3. Measure absorption (or percentage transmission) of the known concentrations

  4. Plot a calibration curve (x-axis =concentration, y-axis = absorption)

  5. Measure absorption of the unknown concentration and read of calibration curve to find the measured concentration

  
• Colorimetry can be used as an accurate alternative to a titration, for finding the concentration of transition metal ions in a solution