Common exam questions (A,B,S)

• Adding excess solid procedure: Measure out a fixed volume of acid, add excess solid to the acid until no more effervescence is observed, filter the mixture to remove excess reactant, heat the filtrate until saturated and allow it to cool, collect the crystals and dry between two pieces of filter paper.
• safety precaution for heating: control the intensity of heat on the bunsen burner to prevent vigorous boiling or sputtering of solution
• Key source of error (adding excess): some of the solution is lost during filtration, thus the yield of crystals will be lower than expected
• key source of error (adding excess): during heating, not all of the solution would crystalize, thus a lower yield of salt will be produced
• to obtain a pure and dry sample of an insoluble salt, rinse the salt with a little cold, deionised water and pat dry between two pieces of filter paper
• precipitation procedure: pour a fixed amount of reactant into a test tube, then pour the same amount of the other reactant into the test tube and mix, fold a piece of filter paper and place it into a funnel on top of a flask, filter the mixture, rinse the paper with deionised water, pat dry with another piece of filter paper, and the pure sample is obtained
• titration procedure: pour the acid into the burette, ensure there are no air bubbles at the tip of the burette, pipette 25.0cm3 of the solution into a conical flask, add 2-3 drops of methyl orange indicator to the conical flask, add the acid from the burette and swirl the flask after each drop, stop adding the acid once one drop changes the colour of the indicator. Record the final burette reading, repeat the titration 2 more times to obtain an average reading, repeat the titration without the indicator, heat the mixture in an evaporating dish and collect a pure, dry sample of salt crystals
• Precautions (titration): ensure the funnel is taken out from the top of the burette before beginning titration to prevent excess acid dripping from the funnel changing the reading of the burette
• Precaution (titration): ensure the conical flask is rinsed with only deionised water, the burette is rinsed with deionised water then the acid and the pipette is rinsed with deionised water and then the reactant
• Drawing of table (titration): 1. final burette reading/cm3, 2. initial burette reading/cm3, 3. volume of acid used/cm3, 4. best titration results