OCR A Jun 2010 Paper 5 Q3

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The chemicals that we call acids have been known for thousands of years. However, modern theories of acids have been developed comparatively recently. It wasnt until the early 1900s that the concept of dissociation became accepted by the scientific community and the concept of pH was introduced. A student carried out a series of experiments with acids and alkalis. (a) Propanoic acid, CH3CH2COOH, is a naturally occurring weak acid. The equation for the dissociation of propanoic acid is shown below. CH3CH2COOH(aq) H+(aq) + CH3CH2COO(aq) The student wanted to prove that propanoic acid is a weak acid. The student had access to a pH meter and 0.100 mol dm3 propanoic acid. Explain how the student could prove that propanoic acid is a weak acid by taking a single pH measurement. Show how the student could then calculate the acid dissociation constant, Ka, for propanoic acid.[4] (b) The student measured the pH of a solution of sodium hydroxide at 25 C. The measured pH was 13.46. Calculate the concentration of the aqueous sodium hydroxide.OCR 2010 concentration =mol dm3 [2]<br />
 (c) A student made a buffer solution by mixing an excess of propanoic acid to an aqueous solution of sodium hydroxide at 25 C. This buffer solution contains an equilibrium system that minimises changes in pH when small amounts of acids and alkalis are added. Explain why a buffer solution formed when an excess of propanoic acid was mixed with aqueous sodium hydroxide. Explain how this buffer solution controls pH when an acid or an alkali is added. In your answer you should explain how the equilibrium system allows the buffer solution to control the pH.[7]OCR 2010 Turn over<br />
 (d) A student added nitric acid to propanoic acid. A reaction took place to form an equilibrium mixture containing two acidbase pairs. Complete the equilibrium below and label the two conjugate acidbase pairs. HNO3 + CH3CH2COOH+[2] (e) Finally, the student reacted an aqueous solution of propanoic acid with a reactive metal and with a carbonate. (i) Write an equation for the reaction of aqueous propanoic acid with magnesium.[1] (ii) Write an ionic equation for the reaction of aqueous propanoic acid with aqueous sodium carbonate.[1] [Total: 17]OCR 2010<br />

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