SASS Chem P3 Possible Errors

Page 1 ST ANDREW’S SECONDARY SCHOOL NAME CLASS CHEMISTRY Chemistry Practical Error Observed in Experiments, Possible Causes and Suggested Improvements Experiment Error Observed Possible Cause Suggested Corrections Thermal decomposition of hydrated salts Mass loss is too large Loss of product Cover the crucible with a lid; Heat the substance slowly Mass loss is too small Incomplete reaction Increase the reaction temperature or reaction time Combustion of a reactive metal Mass gain is too large Absorption of H2O or CO2 after combustion Cool product in desiccator; Measure mass of product quickly Mass gain is too small Incomplete reaction Increase the reaction temperature or reaction time Determining the molar mass of an unknown gas Mr is too high Gas leaks out Use airtight connection or a gas syringe Mr is too low Air leaks in Product obtained in a reaction Yield > 100% Moist or impure product Dry the product; Recrystallise the product; Redistill the product Yield is too low Incomplete reaction Increase the reaction temperature or reaction time Loss of product Modify or change the method Acid–base titration with an indicator Calculated concentration is inaccurate: either too low or too high Incorrect choice of indicator Change to a suitable indicator; Use a pH–meter (datalogger) ‘Old’ or wrong solution used as titrant Prepare and use fresh solution for the titrant Too much indicator used Add only two drops of indicator Heat change experiments using Styrofoam cups ∆H determined is too low (Excess) heat loss to the surrounding Nest cup in empty beaker; Use a lid for the cup; Increase the volume of solutions used ∆H determined is inaccurate Solutions used are too concentrated (affect the mass of solution used in calculation) Dilute the solutions used Rate of reaction Mass loss is too large Evaporation or loss of solvent used Loosely fit a ball of cotton at the mouth of conical flask Volume of gas formed is too small Gas leak or wrong collection method Use airtight connection or a gas syringe