Revision Exercise for Practical Test ANSWER Chemistry SK 015 Session 2023/2024 Madam Ummi Mansor Chemistry Unit, KMNS Answer the following questions. 1. What is titration? A technique used to determine the concentration of a solution using another solution of known concentration. 2. What is a standard solution? A solution of precisely @ accurately @ exactly known concentration, used to determine the concentration (molarity) of the other substances such as solution in titration. 3. What is an end point? The point in a titration at which indicator changes colour. 4. What is an equivalence point? The point in a titration at which the titrant and analyte have reacted completely according to the stoichiometric ratio. 5. What is a solution that is used to detect the end point of titration? An indicator. 6. How does the end point of titration is detected? When the indicator changes colour. 7. What is the purpose of taking a gross reading during titration? As a reference to estimate the volume of titrant needed @ to prevent from overshooting the volume of titrant needed @ to slow down the titration near end point @ to get rough volume of titrant needed. 8. Following gross reading, titration is repeated at least three times. Why is this necessary? To achieve the required precision of results. 9. State three precautions that must be taken during an experiment involving titration and give the reason. Precaution Reason Take the bottom of meniscus that aligns with calibration mark at eye level as the reading. To ensure the accuracy of burette/pipette reading. Ensure that there are no air bubbles trapped in the burette tip. The presence of air bubbles will affect the volume of titrant used in the titration. Upon reaching the end point, titrate slowly until a pale pink colour persists for more than 30 seconds. To ensure the amount of titrant/base added is just enough to neutralise the analyte/acid. A white tile is placed beneath the conical flask. For clear detection of colour change of indicator. During titration, the conical flask is swirled consistently. To ensure the titrant and analyte are mixed and react thoroughly. About 2 cm below the calibrated mark of volumetric flask, distilled water is added dropwise using a dropper. To prevent from overshooting the mark. * Any three. Name : Practicum :
Revision Exercise for Practical Test ANSWER Chemistry SK 015 Session 2023/2024 Madam Ummi Mansor Chemistry Unit, KMNS 10. In a laboratory, a student carried out an experiment to determine the concentration of a stock solution of sulphuric acid, H2SO4. The following are the procedures of the experiment: Step 1 : Rinse a burette with distilled water, followed by 0.20 M NaOH solution. Step 2 : Fill the burette with the NaOH solution. Ensure there are no air bubbles trapped at the tip. Step 3 : Record the initial burette reading to two decimal places. Step 4 : Pipette 25 mL of H2SO4 solution into a 250 mL conical flask. Step 5 : Add 2 drops of phenolphthalein to the H2SO4 solution. Step 6 : Place a white tile underneath the flask so that any colour change can be clearly observed. Step 7 : Titrate the H2SO4 solution with 0.20 M NaOH from the burette. Swirl the flask continuously. Step 8 : Record the final burette reading to two decimal places. Step 9 : Repeat the titration three times. (a) What is the objective of the experiment? To determine the concentration of a stock solution of sulphuric acid, H2SO4. (b) Write the procedures of the experiment in passive sentences. Step 1 : A burette was rinsed with distilled water, followed by 0.20 M NaOH. Step 2 : The burette was filled with the NaOH solution. It was ensured that no air bubbles trapped at the tip. Step 3 : The initial burette reading was recorded to two decimal places. Step 4 : 25 mL of H2SO4 solution was pipetted into a 250 mL conical flask. Step 5 : 2 drops of phenolphthalein was dropped to the H2SO4 solution. Step 6 : A white tile was placed underneath the flask so that any colour change can be clearly observed. Step 7 : The H2SO4 solution was titrated with 0.20 M NaOH from the burette. The flask was swirled continuously. Step 8 : The final burette reading was recorded to two decimal places. Step 9 : The titrations was repeated three times. (c) Complete the table of results provided below: Burette reading (mL) Gross I II III Final reading 18.90 39.00 19.30 28.00 Initial reading 0.00 19.90 0.30 9.00 Volume of NaOH used 18.90 19.10 19.00 19.00 (d) Calculate the average volume of NaOH used in the titration. = + + 3 = 19.10 + 19.00 + 19.00 3 = 19.03 (e) Write a balanced equation for the reaction between H2SO4 and NaOH. H2SO4 (aq) + 2 NaOH (aq) → Na2SO4 (aq) + 2 H2O (l) (f) Based on the equation in (e), calculate the molarity of the H2SO4 solution. = , where a = acid and b = base = 1 2 where a = H2SO4 and b = NaOH (25 ) (0.20 )(19.03 ) = 1 2 = 0.07612 (g) Why does the average volume of NaOH is used to calculate the molarity of the H2SO4 solution? To obtain a precise @ accurate result as only one value of NaOH volume is used in the calculation.