2019 DHS Prelim H2 Chem P3 QP

© DHS 2019 This paper consists of 16 printed pages. [Turn Over Name: Index Number: Class: DUNMAN HIGH SCHOOL Preliminary Examination 2019 Year 6 H2 CHEMISTRY 9729/03 Paper 3 Free Response 25 September 2019 2 hours Additional Materials: Data Booklet Answer Booklet INSTRUCTIONS TO CANDIDATES 1 Write your name, index number and class on this cover page. 2 Write your answers on the separate answer booklet provided. 3 Write in dark blue or black pen. 4 You may use an HB pencil for any diagrams or graphs. 5 Start each question on a fresh page. *[Marks will be deducted if you fail to do so.] 6 Do not use staples, paper clips, glue or correction fluid. Section A 7 Answer all questions. Section B 8 Answer one question. The number of marks is given in brackets [ ] at the end of each question or part question. You are advised to show all workings in calculations. You are reminded of the need for good English and clear presentation in your answers.

2 © DHS 2019 9729/03 Section A Answer all the questions from this section. 1 To facilitate better plant growth, there is widespread use of fertilisers and soil additives in the agricultural sector. (a) Agricultural lime is a soil additive used to increase soil pH so as to facilitate uptake of plant nutrients such as nitrogen and phosphorous. It is usually made up of a combination of calcium carbonate and magnesium carbonate. A manufacturer claims that a 2.00 g sample of agricultural lime contains 92 % calcium carbonate (Mr = 100.1) and 8 % magnesium carbonate (Mr = 84.3) by mass. To verify the manufacturer’s claim, the following steps were carried out: Step 1 The solid carbonat e mixture was dissolved in 250 cm3 of 0.25 mol dm–3 hydrochloric acid. Step 2 A 25 cm 3 aliquot of this resultant solution was titrated with 0.14 mol dm–3 potassium hydroxide. It was found that 12.50 cm 3 of potassium hydroxide was required for complete neutralisation. (i) Based on the manufacturer’s claim of 92 % calcium carbonate and 8 % magnesium carbonate by mass, calculate the theoretical amount, in moles, of each carbonate present in the sample of agricultural lime. [1] (ii) Use the information above to calculate the amount of HC l that reacted with the carbonate mixture in step 1. [2] (iii) The manufacturer’s claim is considered valid if the difference between the actual and theoretical total amount of carbonate present in the sample is less than 0.0010 mol. Using your answers in (a)(i) and (a)(ii), determine if the manufacturer’s claim is valid. [2] (iv) When solid magnesium chloride is dissolved in water, it produces a slightly acidic solution of pH 6.5. Using appropriate equation(s), explain the above observation. [2]

3 © DHS 2019 9729/03 [Turn Over (v) However, t