SNGS 2024 Chem Prelim Paper 3

[Turn over Name: _________________________ ( ) Class: _________ 2024 PRELIMINARY EXAMINATION GENERAL CERTIFICATE OF EDUCATION ORDINARY LEVEL CHEMISTRY 6092/03 Paper 3 Practical 6 August 2024 1h 50 minutes _____________________________________________________________________ READ THESE INSTRUCTIONS FIRST Write your name, register number, and class on all the work you hand in. Give details of the practical shift and laboratory where appropriate, in the boxes provided. Write in dark blue or black pen. You may use a soft pencil for any diagrams or graphs. Do not use highlighters, glue, correction fluid or correction tape. Answer all questions. The use of a scientific calculator is expected, where appropriate. You are reminded of the need for clear presentation in your answers. Qualitative Analysis Notes are printed on page 10. The number of marks is given in brackets [ ] at the end of each question or part question. Shift Laboratory For Examiner’s Use Question 1 (15) Question 2 (11) Question 3 (9) Question 4 (5) Total (40) _____________________________________________________________________ This document consists of 10 printed pages.

2 1 Citric acid is a carboxylic acid found in lemon juice. The equation for the reaction between citric acid, H3C6H5O7, and potassium hydroxide, KOH, is shown below. 3KOH + H3C6H5O7 → K3C6H5O7 + 3H2O The mass of citric acid dissolved in 500 cm3 of an aqueous solution can be determined by titration with aqueous potassium hydroxide. Methyl orange is used to determine the end-point of the titration. Read all the instructions below carefully before starting the experiment in Question 1. P is 0.100 mol/dm3 aqueous potassium hydroxide. Q is aqueous citric acid. (a) Fill the burette with P. Use the pipette to transfer 25.0 cm3 of Q into a conical flask. Add a few drops of the methyl orange to the solution in the conical flask. Add P from the burette, swirling the flask consistently. Record your titration results in the space provided. Repeat the titration as many times as you consider necessary to achieve consistent results. Results [5] (b) Use your titration results to obtain the average volume of P used. Show clearly how you obtained this volume. average volume of P ……………………….[1]

3 (c) Use your results from (b) to calculate the number of moles of aqueous potassium hydroxide in the average volume of P used. moles of aqueous potassium hydroxide ………………………..[1] (d) Calculate the number of moles of citric acid in 25.0 cm3 of Q. moles of citric acid in 25.0 cm3 of Q is ………………………[1] (e) Calculate the concentration of citric acid in Q in mol/dm3. conc