Practical I: Acid-base equilibrium & pH of solutions Aims/Objectives: 1. To determine the pH range where the indicator changes colour. 2. To identify the suitable indicators for different titrations. 3. To identify the unknown acid. 4. To determine acid dissociation constant, Ka and pKa for the unknown acid. Introduction: Titration process is used in an acid-base experiment in order to determine the concentrations of solutions of acids and bases. Through the titration process, we are able to identify physical changes to the mixture such as the colour change to indicate the end point of the experiment. For example, the colour changes of phenolphthalein from colourless to pink and methyl orange from red to orange and subsequently yellow. Acids produce hydrogen ions and bases produce hydroxide ions. This causes the indicator to change colour due to the colour difference from the undissociate molecules. Strong acids and strong bases are strong electrolytes and are assumed to ionize completely in the presence of water. Weak acids however, only ionize to a limited extend in water. Any weak or strong acids when in contact with any weak or strong alkali will start to undergo neutralization regardless of their volume. When an indicator which is present in the acid-base mixture and have experienced colour change, it indicates that the mixture is in right proportions to neutralize each other and is also known as the equivalence point. This experiment has to be carried out carefully