VJC_H2_CHEM_P3 Prelim

 VJC 2011 9647/03/PRELIM/11 [Turn over 2 1 Use of the Data Booklet is relevant to this question. Cyanidin is a natural pH indicator that can be found in many red berries, including grapes, blackberry and cherry. Aqueous solutions of cyanidin change colour with pH, appearing red at pH < 3, violet at pH 7-8, and blue at pH > 11. In addition, cyanidins are antioxidants that can scavenge free radicals and counteract ageing caused by oxidative damage in our body tissues and cells. Cyanidin Cyanidin (CyH) is a weak monoprotic acid. In one experiment, 25.0 cm 3 of cyanidin was titrated against 0.10 mol dm –3 of dilute NaOH at 25°C. The following titration curve was obtained using a data logger. (a)(i) Explain what is meant by a weak monoprotic acid, with reference to CyH. (ii) Calculate the molar concentration of the cyanidin solution. (iii) Given that the solution of cyanidin is only 0.8% dissociated into ions, calculate the value of K a for cyanidin, stating clearly its units. (iv) Explain, with the aid of an appropria te equation, why the pH at end-point B is greater than 7. × 26.20 Volume of NaOH /cm3 A B pH

 VJC 2011 9647/03/PRELIM/11 [Turn over 3 (v) Calculate the pH value at end-point B. Hence, state the colour change observed at the end-point. [10] (b) An aqueous solution of cyanidin CyH and Cy- can act as a buffer. (i) Copy the titration curve onto your answer script and label I – the buffer region II – the point corresponding to maximum buffer capacity by indicating the pH and volume of NaOH used. (ii) Explain, with the help of equations , how an aqueous mixt ure of CyH and Cy - can control pH when relatively sm all amounts of acid or base are added to the solution. (iii) In a sample of cherry juice at 25°C, the concentration ratio of Cy - to CyH was found to be 3 to 1. Using the Ka value determined above, calculate the pH value of the cherry juice. (iv) Sam tried to dilute the same cherry juice sample by adding 100 cm 3 of deionized water at 25°C. Predict the pH of the resulting sample. [6] (c) Cyanidin can be synthesized via a number of steps from either of the two molecules shown below: Molecule P Molecule Q (i) Which molecule P or Q will have a higher boiling point? Explain your answer. (ii) Draw the structur es of all the possibl e organic products when P and Q react separately with chlorine in trichloromethane. [4] [Total: 20]

 VJC 2011 9647/03/PRELIM/11 [Turn over 4 2 This question is about elements and their compounds in Period 3, Group IV and Group VII. (a) The elements in Period 3 range from metals on the left of the Periodic Table to non-metals on the right. By describing one physical property of the element and one chemical property of its chloride, explain why phosphorus can be regarded