VJC_H2_CHEM_P3_ANS_FINAL Prelim

 VJC 2011 9647/03/PRELIM/11 [Turn over 1 Victoria Junior College 2011 H2 Chemistry Prelim Exam 9647/3 Suggested Answers 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 soluti ons 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. × 26.20 Volume of NaOH/ cm3 A B pH

 VJC 2011 9647/03/PRELIM/11 [Turn over 2 (a)(i) Explain what is meant by a weak monoprotic acid, with reference to CyH. [1] A weak monoprotic acid is an acid that dissociates partially to donate only one proton per acid molecule. i.e. CyH Cy + H+ (ii) Calculate the molar concentration of the cyanidin solution. [1] Since n CyH = nNaOH, Conc. of cyanidin = 0.25 20.2610.0  = 0.105 mol dm3 (iii) Given that the solution of cyanidin is onl y 0.8% dissociated into ions, calculate the value of Ka for cyanidin, stating clearly its units. [2] [H+] = [Cy] = 0.008 x 0.105 = 8.40 x 10-4 mol dm3 Ka = ][ ]][[ CyH HCy  K a = 105.0 )1040.8( 24 = 6.72 x 10-6 mol dm3 [OR 6.77 x 10 -6 if dissociation of CyH is considered] (iv) Explain, with the aid of an appropriate equation, why the pH at end-point B is greater than 7. [2] At end-point B, only the conjugate base of the weak acid, Cy is present. Cy + H2O CyH + OH Cy hydrolyses in water to produce excess OH ions. Hence, pH > 7. (v) Calculate the pH value at end-point B. Hence, state the colour change observed at the end-point. [4] Conc. of salt = 20.260.25 20.2610.0   = 0.0512 mol dm3 Kb = ][ ]][[   Cy OHCyH = a w K K = 6 14 1072.6 10    = 1.49 x 109 mol dm3 9 22 1049.10512.0 ][ ][ ][      OH Cy OH [OH -] = 8.73 x 106 pOH = - log 10 [OH-] = 5.06 pH = 14 – pOH = 8.94 Colour change at end-point: violet to blue.

 VJC 2011 9647/03/PRELIM/11 [Turn over 3 (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 added. [1] (ii) Explain, with the help of equations, how an aqueous mixture of CyH + and Cy can control pH when relatively small amounts of acid or base is added to the solution. [3] When a small amount of acid is add