OCR A Jan 2012 Paper 5 Q4

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This question looks at acids, bases and buffer solutions. 10 (a) Nitric acid, HNO3, is a strong BrnstedLowry acid. Nitrous acid, HNO2, is a weak BrnstedLowry acid with a Ka value of 4.43104 mol dm3. (i) What is the difference between a strong acid and a weak acid?[1] (ii) What is the expression for the acid dissociation constant, Ka, of nitrous acid, HNO2? (iii) Calculate the pH of 0.375 mol dm3 nitrous acid, HNO2. Give your answer to two decimal places. [1] pH =[2] (iv) A student suggests that an acidbase equilibrium is set up when nitric acid is mixed with nitrous acid. Complete the equation for the equilibrium that would be set up and label the conjugate acidbase pairs. HNO3 + HNO2+[2]OCR 2012<br />
 (b) Calcium hydroxide, Ca(OH)2, is a strong BrnstedLowry base. 11 (i) Explain what is meant by the term BrnstedLowry base.[1] (ii) Calculate the pH of 0.0400 mol dm3 Ca(OH)2. Give your answer to two decimal places. pH =[3] (c) Aqueous calcium hydroxide is added to nitrous acid, HNO2. Write the overall equation and the ionic equation for the reaction that takes place. overall:ionic:[2]OCR 2012 Turn over<br />
 12 (d) Carbonic acid, H2CO3, is a weak BrnstedLowry acid formed when carbon dioxide dissolves in water. Healthy blood is buffered to a pH of 7.40. The most important buffer solution in blood is a mixture of carbonic acid and hydrogencarbonate ions, HCO3 . (i) Explain how the carbonic acidhydrogencarbonate mixture acts as a buffer in the control of blood pH. In your answer you should explain how equilibrium allows the buffer solution to control the pH.[5]OCR 2012<br />
 13 (ii) Healthy blood at a pH of 7.40 has a hydrogencarbonate : carbonic acid ratio of 10.5 : 1. A patient is admitted to hospital. The patients blood pH is measured as 7.20. Calculate the hydrogencarbonate : carbonic acid ratio in the patients blood. [5] [Total: 22]OCR 2012 Turn over<br />

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