NJC Planning notes

National Junior College SH1 H2 Chemistry 4 Some Possible Practical/Planning Exercises Involving Titration Types of Expts Some Examples To ensure reliability Remarks (a) Acid-Base (b) Redox: MnO4− + reducing agents like H2O2, Fe2+, NO2− and C2O42−. (c) Iodometric titrations: I2 (formed by analyte) + reducing agent S2O32− (d) Back Titration: HCl is added in excess to react with and dissolve the insoluble carbonate compound. The excess HCl is then titrated against a standard solution of sodium hydroxide • To determine the formula of a compound • To determine the unknown concentration / % by mass of a reactant in a given mixture • To determine the solubility product of sparingly soluble salts. • Dropwise addition of titrant (solution in burette) when approaching the end-point. • Repeat titration until results are consistent • (two titres within ± 0.10 cm3) For (a) – (c): ALL titrations • For a given solid sample or aqueous sample of high concentration, preparation of its bulk / diluted solution in a graduated flask is required so that titration can be repeated to obtain consistent results. For (a): Acid-Base Titrations • only 2–3 drops of indicator (eg. phenolphthalein, thymol blue or methyl orange) are required. For (b): Redox Titrations • Add excess dilute H2SO4 (not HNO3 or HCl) to provide the acidic medium required for MnO4− titration. • No indicator is required as MnO4− is self-indicating. For (c): Iodometric titrations normally involve two steps: (i) An oxidising agent + KI to produce iodine, I2. (ii) The I2 produced is then titrated against sodium thiosulfate(VI), Na2S2O3. 1 cm3 of starch solution serves as the indicator and is only added near the end-point when most of the I2 has been reacted away by Na2S2O3. For (d): Back Titrations • For substances that are insoluble in water. A bulk solution of the sample is prepared through the use of a reaction (usually acid-base reaction). The unreacted excess reagent that is used to dissolve the sample is then titrated against a standard solution.

National Junior College SH1 H2 Chemistry 5 TITRATION 1 Standard Solution • Standard solution: A solution of accurately known concentration. • Primary standard solution: A solution of known concentration prepared by dissolving an accurately known mass of solute in a known volume of solution. Such a solution can only be prepared using a solute which has the following properties: (a) is available in high purity (b) forms a stable solution which is unaffected by air or light (c) does not have the following properties: volatile2, deliquescent3, hygroscopic4 • Secondary standard solution (bulk solution): A solution whose concentration is determined by titration with another standard solution (usually a primary standard solution). 2 Preparation of a Standard / Bulk Solution Depending on the reagent given, a standard solution can either be prepared by dissolving an accurately measured amount of solid or concentrated aqueous sample and making up the solution with deionised water in a standard graduated / volumetric flask. (A) Preparation of a Standard / Bulk Solution from a Solid sample *To memorise standard phrasing shown below: 1. Weigh accurately about _______ g of the solid in a clean and dry weighing bottle. 2. Transfer solid to a small clean beaker. Reweigh the weighing bottle and the residual solid and calculate the actual mass of solid transferred. 3. Record the readings in the table below: Mass of solid used to prepare standard solution Mass of weighing bottle / g s Mass of weighing bottle with solid / g t Mass of weighing bottle and residual solid / g u Mass of solid transferred to small beaker / g (t – u) = m 4. Dissolve the solid with some deionised water in a small beaker. 5. Transfer the solution and ALL of the washings into a 100 cm3 (or 250 cm3) graduated / volumetric flask. 6. Make up to the mark with deionised water, stopper and shake well to ensure a homogeneous solution. 2 Liquid substance easily evaporates into gaseous state at a low temperature due to its low boiling point. 3 the process by which a substance absorbs moisture from the atmosphere until it dissolves in the absorbed water and forms a solution. 4 A substance that readily attracts water from its surroundings, through either absorption or adsorption.

National Junior College SH1 H2 Chemistry 6 (B) Preparation of a Standard / Bulk Solution from a concentrated aqueous solution *To memorise standard phrasing shown below: 1. Measure ______ cm3 of solution using a burette (or pipette) into a 100 cm 3 (or 250 cm3) graduated / volumetric flask. 2. Make up to the mark with deionised water, stopper and shake well to ensure a homogeneous solution. Preparing a Standard / Bulk Solution for Use in Titration Procedure Step Remarks (a) (b) (c) (d) (e) (f) (g) (h) (a) Accurate mass of the solid measured by electronic balance. Weighing bottle must be clean and dry. The beaker and glass rod are to be washed with tap water and rinsed with deionised water before use. Wipe dry with tissue. (b) Solid is transferred to a small, clean beaker. The mass of the weighing bottle and residual solid is measured again to calculate the actual mass transferred. (c) Stir with glass rod to ensure all solid is dissolved in a minimal volume of water. (d) To avoid spillage, the concentrated solution is transferred from the beaker to the volumetric flask via a filter funnel, using the glass rod to direct the flow of the solution. Do not rinse the volumetric flask with solution before transfer. (e) Wash the glass rod and inner walls of the beaker with minimal volume of deionised water. (f) Transfer washings to volumetric flask. Repeat steps (e) and (f) 2 times. (g) Top up with deionised water till it is 0.5 cm below the calibration mark and use a dropper to adjust to meet the mark. (h) Stopper, invert and shake the flask to attain a homogeneous solution.

National Junior College SH1 H2 Chemistry 7 3 Use of Pipette 1. Wash the pipette by rinsing its interior with tap water followed by deionised water. 2. Dry the exterior of the pipette. (to prevent introduction of water into solution when step 3 is done) 3. Rinse the pipette with the solution to be used to ensure solution is not diluted by water in pipette. 4. Hold the upper end of the pipette to insert the pipette filler to avoid breakage. 5. Draw the solution into the pipette using the pipette filler and adjust until the bottom of the meniscus level touches the calibration mark. 6. Ensure that there are no air bubbles trapped in the solution. 7. Wipe off the outside of the pipette with a clean tissue paper. 8. Transfer the solution into a conical flask by using the pipette filler to eject the solution or gently removing the pipette filler to drain the solution. 9. Tilt the conical flask slightly and rotate the pipette tip against the bottom of the conical flask. Do not blow out or force the last drop at the tip as it is already calibrated for. When fitting a pipette into a pipette filler, position the hand at the top of the pipette, instead of below the pipette bulb. Do not push the pipette too deeply into the pipette filler. When removing the pipette from the pipette filler, twist the pipette out gently