Copy of ACSBR 6092 Sec 4 Prelims 2024_P3

Anglo-Chinese School (Barker Road) Preliminary Examination 2024 Secondary 4 Express Chemistry 6092/3 2 1 (a) When sodium carbonate is crystallised from solution, it forms crystal s of hydrated sodium carbonate, Na2CO3•xH2O, where x is the amount of water of crystallisation present. You are going to determine the amount of water of crystallisation in Na2CO3•xH2O. In this question, you will perform a titration and the equation for the reaction in the titration is as follows: Na2CO3 + 2HCl → 2NaCl + CO2 + H2O The data from the titration will then be used to calculate: • the molar concentration of Na2CO3 in Q, • the relative molecular mass, Mr, of Na2CO3•xH2O and hence the value of x. Read all the instructions below carefully before starting the experiment in Question 1. Instructions P is 1.50 mol/dm3 aqueous hydrochloric acid. Q contains 200 g/dm3 of Na2CO3•xH2O in deionised water. (i) 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 indicator to the solution in the conical flask. Add P from the burette, swirling the flask constantly. At the end-point, one drop of P will turn methyl orange from yellow to orange. Record your titration results in the table below. Repeat the titration as many times as you consider necessary to achieve consistent results. Results [5]

Anglo-Chinese School (Barker Road) Preliminary Examination 2024 Secondary 4 Express Chemistry 6092/3 3 (ii) From your titration results, obtain an average volume of P to be used in your calculations. Show clearly how you obtained this volume. average volume of P = …………….……… cm3 [1] (iii) P is 1.50 mol/dm3 aqueous hydrochloric acid. Use your answer from (a)(ii) and the equation for the reaction to c alculate the molar concentration of Na2CO3 in Q. molar concentration of Q = …………………… mol/dm3 [2] (iv) Use your answer from (a)(iii) to calculate the Mr of Na2CO3•xH2O to 1 decimal place; and hence the value of x to the nearest whole number. You must show your working. [Mr: Na2CO3, 106; H2O, 18] Mr of Na2CO3•xH2O = …………….……… x = …………….……… [2]

Anglo-Chinese School (Barker Road) Preliminary Examination 2024 Secondary 4 Express Chemistry 6092/3 4 (v) Before the start of the experiment, a student accidentally rinsed the pipette with solution P instead of solution Q, before filling it up with solution Q. Describe and explain the effect this would have on the amount of water of crystallisation. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. ………………………………………………………………………………………………….. …………………………………………………………