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OCR A Jun 2017 Paper 2 Q2
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![2* A student was asked to carry out an experiment to determine the initial rate of reaction of zinc and hydrochloric acid. The student plans to collect a total of about 72 cm3 of hydrogen at RTP and to use an excess of zinc. The student selects the following apparatus:the apparatus shown in the diagram 100 cm3 measuring cylinder stop clock 2 decimal place balance 100 cm3 gas syringe Outline how the student could carry out the experiment and explain how the results could be processed graphically. Show all working in your calculations.[6]<br />](https://learnah.org/wp-content/uploads/2018/08/June_2017_QP_-_Paper_2_OCR_Chemistry_A_AS-Level_2.jpg)
![H032/02 Mark scheme June 2017 Question 2* Answer Please refer to marking instructions on page 5 of mark scheme for guidance on how to mark this question. Level 3 (56 marks) All three scientific points are covered in detail and explained thoroughly. The method is logically structured and clear calculations are shown for an appropriate mass of metal and suitable volume of acid. The drawing of a tangent and determination of the gradient is communicated well. Level 2 (34 marks) Candidates cover all three scientific points but explanations may be incomplete. OR Two of the scientific points are described thoroughly with no omissions. There is a line of reasoning presented with some structure. The information presented is relevant and supported by some evidence. e.g. there are clear calculations to justify mass and acid volume supported by some working and units; a simple description for determining initial rate related to tangent but no detail of how to measure gradient Level 1 (12 marks) Marks Guidance Indicative scientific points 1. Methodmeasure mass of (excess) zinc (using 2 decimal place balance)measure volume of hydrochloric acid (using measuring cylinder)mix zinc and acid in flaskmeasure gas volume at time intervals 2. Calculationsmoles of hydrogen 72/24000 = 0.00300 molminimum mass of zinc 0.00365.4 = 0.20 gmoles of hydrochloric acid Zn + 2HClZnCl2 + H2 0.003002 = 0.00600 molvolume/concentration of acid If [HCl(aq)] = 0.1 mol dm3 appropriate volume of acid = 0.0061000/0.1 = 60 cm3 If [HCl(aq)]0.3 mol dm3, too low ( 20 cm3) If [HCl(aq)]0.03 mol dm3 too high ( 200 cm3) 3. Processing resultsPlot a graph of volume against timeDraw a tangent at t = 0Gradient of tangent = initial rateGradient = volume/time<br />
H032/02 Question Mark scheme June 2017 Answer Marks Guidance There is a description based on at least two of the main scientific points OR The candidate explains one scientific point thoroughly with few omissions. There is an attempt at a logical structure with a line of reasoning. The information is in the most part relevant. e.g. add zinc and acid and measure volume (no mass, volume or time intervals); calculations that have little structure, absent units and little working. 0 marks No response or no response worthy of credit. Total Level 0<br />](https://learnah.org/wp-content/uploads/2018/08/June_2017_MS_-_Paper_2_OCR_Chemistry_A_AS-Level_2-0.jpg)
