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OCR A2 CHEMISTRY A

TOPIC 5.1.3

ACIDS AND BASES

BOOKLET OF PAST EXAMINATION QUESTIONS

TOTAL 256 MARKS

1.	(a)	(i)	Define the term Brφnsted-Lowry acid.
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(ii)	What is meant by the term strong when describing an acid?
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(iii)	Give the value of the ionic product of water, K_w, measured at 298K, and state its units.
Value.................................................................................................................
Units..................................................................................................................
(4) 
(b)	At 298 K, 25.0 cm^3 of a solution of a strong acid contained 1.50 × 10^–3 mol of hydrogen ions.
(i)	Calculate the hydrogen ion concentration in this solution and hence its pH.
Hydrogen ion concentration.............................................................................
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pH...................................................................................................................... 
(ii)	Calculate the pH of the solution formed after the addition of 50.0 cm^3 of 0.150 M NaOH to the original 25.0 cm^3 of acid.
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(8)

(c)	A solution of a strong acid was found to have a pH of 0.5
(i)	Calculate the hydrogen ion concentration in this solution.
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(ii)	Calculate the volume of water which must be added to 25.0 cm^3 of this solution to increase its pH from 0.5 to 0.7
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(5)
(Total 17 marks)

2.		The pH of a 0.15 M solution of a weak acid, HA, is 2.82 at 300 K.
(i)	Write an expression for the acid dissociation constant, K_a, of HA, and determine the value of K_a for this acid at 300 K, stating its units. 
Expression for K_a ............................................................................................
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Value of K_a......................................................................................................
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 (ii)	The dissociation of HA into its ions in aqueous solution is an endothermic process. How would its pH change if the temperature were increased? Explain your answer.
Effect on pH......................................................................................................
Explanation.......................................................................................................
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(8)
(Total 8 marks)

3.	(a)	Explain the terms acid and conjugate base according to the Brønsted-Lowry theory.
Acid .............................................................................................................................
Conjugate base ...........................................................................................................
(2)
(b)	For each of the following reactions, give the formula of the acid and of its conjugate base.
(i)	NH_3 +HBr → NH + Br^–
Acid ..............................................    Conjugate base .......................................
(1)
(ii)	H_2SO_4 + HNO_3 → HSO + H_2NO
Acid ..............................................    Conjugate base .......................................
(1)

(c)	(i)	Write an equation to represent the dissociation of water.
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(1)
(ii)	Give the expression for the equilibrium constant, K_c, for the reaction in (c)(i) and use this to derive the expression for the ionic product of water, K_w.
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(3)
(iii)	The ionic product of water is 2.92 × 10^–14 mol^2 dm^–6 at 313K. Calculate the pH of water at this temperature.
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(4)
(iv)	Given that the pH of water is 7.00 at 298 K, state whether the dissociation of water is endothermic or exothermic. Give a reason for your answer.
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(2)
(Total 14 marks)

4.	(a)	Give the Brønsted–Lowry definition of a base. State the essential feature of an acid-base reaction in aqueous solution, writing an ionic equation to illustrate your answer.
Definition of a base ....................................................................................................
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Essential feature .........................................................................................................
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Equation .....................................................................................................................
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(3)
(b)	Explain what is meant by the term weak when applied to acids and bases.
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(1) 
(c)	In aqueous solution, the weak acid propanoic acid, CH_3CH_2COOH(aq), produces propanoate ions CH_3CH_2COO^–(aq). Write an expression for the acid dissociation constant, K_a, of propanoic acid and state its units.
Expression for K_a ......................................................................................................
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Units of K_a .................................................................................................................
(2) 
 (Total 6 marks)

7.	(a)	Define the term Brønsted-Lowry acid.
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(1)
 (b)	Write an equation for the reaction between gaseous hydrogen chloride and water.
State the role of water in this reaction, using the Brønsted-Lowry definition.
Equation .....................................................................................................................
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Role of water ..............................................................................................................
(2)
(c)	Write an equation for the reaction between gaseous ammonia and water. State the role of water in this reaction, using the Brønsted-Lowry definition.
Equation .....................................................................................................................
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Role of water ..............................................................................................................
(2)

(d)	The ion H_2NO is formed in the first stage of a reaction between concentrated nitric acid and an excess of concentrated sulphuric acid. In this first stage the two acids react in a 1:1 molar ratio. In the second stage, the H_2NO ion decomposes to form the nitronium ion, NO. Write equations for these two reactions and state the role of nitric acid in the first reaction.
Equation for formation of H_2NO ............................................................................
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Role of nitric acid .......................................................................................................
Equation for formation of NO .................................................................................
(3)
(e)	(i)	Explain the term weak acid.
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(ii)	Write an expression for the acid dissociation constant, K_a, of HA, a weak monoprotic acid.
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(iii)	The value of the acid dissociation constant for the monoprotic acid HX is
144 mol dm^–3. What does this suggest about the concentration of undissociated HX in dilute aqueous solution?
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(iv)	State whether HX should be classified as a strong acid or a weak acid.
Justify your answer.
Nature of HX ....................................................................................................
Justification ......................................................................................................
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(5)
(Total 13 marks) 

9.	(a)	Phenol is a weak acid. The dissociation of phenol in aqueous solution is represented by the following equation:
	C_6H_5OH(aq) + H_2O(l)  H_3O^+(aq) + C_6H_5O^– (aq)
What is meant by the term weak acid?
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(1) 
(b)	(i)	Write an expression for the acid dissociation constant, K_a, for phenol.

(1)
(ii)	Write an expression linking K_a with pK_a.
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(1)
(iii)	The value of the acid dissociation constant, K_a, for phenol is 1 × 10^–10 mol dm^–3.
Calculate the pK_a value of phenol.
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(1) 
(iv)	Ethanoic acid is a stronger acid than phenol. State whether the pKa value for ethanoic acid will be greater or smaller than that of phenol.
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(1)
(Total 5 marks)

10.	(a)	(i)	Define the term pH.
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(ii)	Write an expression for the dissociation constant K_a for the weak acid HX.
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(iii)	For HX, K_a = 4.25 × 10^–5 mol dm^–3. Calculate the pH of a 0.45 M solution of this acid.
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(6)
(b)	In a 0.25 M solution, a different acid HY is 95% dissociated.
(i)	Calculate the pH of this solution.
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(ii)	Calculate the value of K_a for the acid HY.
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 (6)
(Total 12 marks)

11.	(a)	Define pK_a
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(1) 
(b)	Calculate the pH of a 0.52M aqueous solution of the weak monoprotic (monobasic) acid HX (pK_a = 3.72).
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(4)
 (Total 5 marks)

12.	The value of the acid dissociation constant, K_a, for ethanoic acid is 1.74 × 10^–5 mol dm^–3 at
298 K.
(a)	(i)	Write an expression for K_a for ethanoic acid.
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(ii)	Calculate the pH at 298 K of a 0.220 mol dm^–3 solution of ethanoic acid.
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(5) 
(Total 5 marks)

14.	(a)	At 50°C, the ionic product of water, K_w, has the value 5.48 × 10^–14 mol^2 dm^–6.
(i)	Define the term K_w
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(ii)	Define the term pH
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(iii)	Calculate the pH of pure water at 50 °C. Explain why pure water at 50 °C is still neutral even though its pH is not 7.
Calculation .......................................................................................................
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Explanation ......................................................................................................
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(5)
(b)	At 25°C, K_w has the value 1.00 × 10^–14 mol^2 dm^–6. Calculate the pH at 25 °C of
(i)	a 0.150 mol dm^–3 solution of sodium hydroxide,
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(ii)	the solution formed when 35.0 cm^3 of this solution of sodium hydroxide is mixed with 40.0 cm^3 of a 0.120 mol dm^–3 solution of hydrochloric acid.
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(8) 

(c)	In a 0.150 mol dm^–3 solution of a weak acid HX at 25 °C, 1.80% of the acid molecules are dissociated into ions.
(i)	Write an expression for K_a for the acid HX.
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(ii)	Calculate the value of K_a for the acid HX at this temperature and state its units.
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(5)
(Total 18 marks)

15.	(a)	By reference to the forces between molecules, explain why ammonia is very soluble in water.
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(2) 
(b)	Aqueous solutions of ammonia have a pH greater than 7.
(i)	Write an equation for the reaction of ammonia with water.
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(ii)	Explain why the pH of a solution containing 1.0 mol dm^–3 of ammonia is less than 14 at 298 K.
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(3)
(c)	An ammonium ion in aqueous solution can behave as a Brønsted–Lowry acid. State what is meant by the term Brønsted–Lowry acid.
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(1)
 (Total 6 marks)

16.	The value of the acid dissociation constant, K_a, for the weak acid HA, at 298 K, is
1.45 × 10^–4 mol dm^–3.
(a)	Write an expression for the term K_a for the weak acid HA.
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(1)
(b)	Calculate the pH of a 0.250 mol dm^–3 solution of HA at 298 K.
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(4)
 (Total 5 marks)

17.	(a)	The pH of a 0.120 mol dm^–3 solution of the weak monoprotic acid, HX, is 2.56 at 298 K.
(i)	Write an expression for the term pH.
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(ii)	Write an expression for the dissociation constant, K_a, for the weak acid HX and calculate its value at 298 K.
Expression for K_a .............................................................................................
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Calculation .......................................................................................................
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(5)
(b)	(i)	Write an expression for the ionic product of water, K_w, and give its value at 298 K.
Expression for K_w ............................................................................................
Value of K_w .......................................................................................................
(ii)	Hence, calculate the pH of a 0.0450 mol dm^–3 solution of sodium hydroxide at 298 K.
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(4)
 (Total 9 marks)

18.	(b)	(i)	   Write an expression for the term pH.
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(ii)	A solution of potassium hydroxide has a pH of 11.90 at 25°C. Calculate the concentration of potassium hydroxide in the solution.
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(4)
 (Total 4 marks)

19.	(a)	A sample of hydrochloric acid has a pH of 2.34
Write an expression for pH and calculate the concentration of this acid.
pH ................................................................................................................................
Concentration ..............................................................................................................
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(2) 
(b)	A 0.150 mol dm^–3 solution of a weak acid, HX, also has a pH of 2.34
(i)	Write an expression for the acid dissociation constant, K_a, for the acid HX.
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(ii)	Calculate the value of K_a for this acid and state its units.
	Calculation .......................................................................................................
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	Units .................................................................................................................
(iii)	Calculate the value of pK_a for the acid HX. Give your answer to two decimal places.
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(5)

(c)	A 30.0 cm^3 sample of a 0.480 mol dm^–3 solution of potassium hydroxide was partially neutralised by the addition of 18.0 cm^3 of a 0.350 mol dm^–3 solution of sulphuric acid.
(i)	Calculate the initial number of moles of potassium hydroxide.
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(ii)	Calculate the number of moles of sulphuric acid added.
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(iii)	Calculate the number of moles of potassium hydroxide remaining in excess in the solution formed.
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(iv)	Calculate the concentration of hydroxide ions in the solution formed.
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(v)	Hence calculate the pH of the solution formed. Give your answer to two decimal places.
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(6)
(Total 13 marks)

20.	In this question, give all pH values to 2 decimal places.
(a)	(i)	Write expressions for the ionic product of water, K_w, and for pH.
K_W = ..................................................................................................................
pH = .................................................................................................................. 
(ii)	At 318 K, the value of K_w is 4.02 × 10^–14 mol^2 dm^–6 and hence the pH of pure water is 6.70
State why pure water is not acidic at 318 K.
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(iii)	Calculate the number of moles of sodium hydroxide in 2.00 cm^3 of 0.500 mol dm^–3 aqueous sodium hydroxide.
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(iv)	Use the value of K_w given above and your answer to part (a)(iii) to calculate the pH of the solution formed when 2.00 cm^3 of 0.500 mol dm^–3 aqueous sodium hydroxide are added to 998 cm^3 of pure water at 318 K.
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(6)

(b)	At 298 K, the acid dissociation constant, K_a, for propanoic acid, CH_3CH_2COOH, has the value 1.35 × 10^–5mol dm^–3.
(i)	Write an expression for K_a for propanoic acid.
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(ii)	Calculate the pH of 0.125 mol dm^–3 aqueous propanoic acid at 298 K.
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(4)
(Total 10 marks)

21.	
(d)	(i)	Calculate the change in pH when 0.250 mol dm^–3 hydrochloric acid is diluted with water to produce 0.150 mol dm^–3 hydrochloric acid.
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(ii)	Calculate the volume of water which must be added to 30.0 cm^3 of 0.250 mol dm^–3  hydrochloric acid in order to reduce its concentration to 0.150 mol dm^–3.
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(4)
(Total 12 marks)

22.	(a)	Give the Brønsted–Lowry definition of an acid.
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(1)
(b)	(i)	Explain the term weak when applied to an acid or a base.
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(ii)	Give an example of a weak base and write an equation involving this weak base to illustrate the explanation you gave in part (i) above. 
Example. ..........................................................................................................
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Equation. .........................................................................................................
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(3)

(c)	In aqueous solution, the weak acid methanoic acid, HCOOH, produces aqueous methanoate ions, HCOO^–(aq).
(i)	Write an equation, including state symbols, for the formation of methanoate ions and H_3O^+ ions in an aqueous solution of methanoic acid.
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(ii)	Identify one substance that acts as a Brønsted–Lowry base in the forward direction, and another in the reverse direction, of the equation you have written in part (i) above.
Base in forward direction. ...............................................................................
Base in reverse direction. ................................................................................ 
(iii)	Write an expression for the acid dissociation constant, K_a, of methanoic acid.
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(4)
(Total 8 marks)

23.	(a)	Write an equation, including state symbols, for the reaction of gaseous hydrogen chloride with water.
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(1)
(b)	Calculate the pH of a 1.26M solution of HCl.
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(2) 
(c)	Suggest a value for the pH of a 1.26M solution of sodium chloride. Explain your answer.
pH. .............................................................................................................................
Explanation. ..............................................................................................................
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(2) 
(d)	Barium hydroxide, Ba(OH)_2, dissociates completely in water according to the equation
	Ba(OH)_2(s) → Ba^2+(aq) + 2OH^–(aq)
	45.0cm^3 of 1.37M barium hydroxide were added to 95.0cm^3 of l.26M hydrochloric acid.
(i)	Calculate the number of moles of H^+ ions in 95.0 cm^3 of 1.26M hydrochloric acid.
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(ii)	Calculate the number of moles of OH^– ions in 45.0 cm^3 of 1.37 M barium hydroxide.
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(iii)	Calculate the pH at 25 °C of the solution formed after mixing the acid and the base.
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(8)
(Total 13 marks)

24.	HA is a weak monoprotic acid which can be used to make buffer solutions.  It dissociates in water according to the equation
	HA(aq)   H^+ (aq) + A^–(aq)
(a)	Explain what is meant by the term weak when applied to an acid.
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(1) 
(Total 1 marks)

25.	(a)	An acid HA has pK_a = 4.20
(i)	Define the term pK_a
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(ii)	Calculate the value of the dissociation constant, K_a, for the acid HA and state its units.
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(iii)	Calculate the pH of a 0.830 M solution of the acid HA.
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(7)
(Total 7 marks)

26.	(a)	State what is meant by the term monoprotic acid and give one example 
Monoprotic acid..........................................................................................................
Example......................................................................................................................
(2) 
(b)	(i)	Define pH.
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(ii)	What is the hydrogen ion concentration in a solution which has pH = - 0.20?
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(2)
(c)	Calculate the pH of the solution formed when 35 cm^3 of 0.12 M NaOH are added to 
25 cm^3 of 0.15M HCl at 25^°C.
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(7)
(Total 11 marks) 

27.	All solutions in parts (a) to (d) below are maintained at 25 °C.
(a)	Write equations to show the reaction of HCl(g) and KOH(s) with water.
Equation for HCl(g) ..................................................................................................
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Equation for KOH(s) .................................................................................................
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(2) 
(b)	Write an expression to define the ionic product, K_w, of water.
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(1)

(c)	Calculate the pH of a 0.0160 M KOH solution and estimate the pH of a 0.100 M KCl solution. Give your reasoning.
	(At 25 °C, the value of K_w is 1.00 × 10^–14 mol^2 dm^–6)
pH of 0.0160 M KOH ................................................................................................
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pH of 0.100 M KCl ....................................................................................................
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(7) 
(d)	A mass, m, of solid KOH is added to 755 cm^3 of 0.0120 M HCl. The pH after this addition is 11.60, measured at 25 °C. The volume of the resulting solution is still 755 cm^3.
(i)	Calculate the number of moles of OH^– ions needed to neutralise exactly the H^+ ions present in the 755 cm3 of 0.0120 M HCl.
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(ii)	Calculate the number of moles of OH^– ions in excess when the pH is 11.60
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(iii)	Use these results to calculate the total number of moles of KOH added.
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(iv)	Hence deduce the value of m.
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(8)
(Total 18 marks)

 28.	(a)	The hydrogen halides all react with water to form acids. Hydrogen fluoride forms a weak acid while the others all form strong acids. Write equations to show the reactions that occur when hydrogen fluoride and hydrogen chloride are dissolved in water.
Hydrogen fluoride and water ......................................................................................
Hydrogen chloride and water .....................................................................................
(2)
(b)	(i)	Define the term pH............................................................................................
(1)
(ii)	Calculate the pH of an aqueous solution of hydrochloric acid containing 0.050 mol dm^–3.

(1) 
(c)	(i)	Write an expression for the dissociation constant, K_a, for hydrofluoric acid.

(1)
(ii)	Calculate the pH of an aqueous solution of hydrofluoric acid of concentration 0.050 mol dm^–3 at 298K, given that K_a =5.6 × 10^–4 mol dm^–3 at 298 K.

(3)
(d)	When hydrogen fluoride is dissolved in pure nitric acid, a reaction takes place that can be represented by the equation:
	HNO_3 + HF → H_2 NO + F^–
	State, with a reason, which reactant acts as a Brønsted-Lowry acid in this reaction and give the formula of its conjugate base.
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(3)
(Total 11 marks)

29.	(a)	When dissolved in water, ethanoic acid acts as a weak Brønsted-Lowry acid.
	CH_3COOH + H_2O  CH_3COO^– + H_3O^+
Explain the terms:
(i)	Brønsted-Lowry acid; ........................................................................................
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(1)
(ii)	weak acid. ..........................................................................................................
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(1)
(b)	(i)	Write an expression for the acid dissociation constant, K_a, for ethanoic acid.

(1)
(ii)	Calculate the pK_a value of aqueous ethanoic acid.
K_a (ethanoic acid) = 1.70 × 10^–5 mol dm^–3 at 25°C

(1) 
(e)	In the gaseous state, some ethanoic acid molecules are dimerised as shown in diagram below. The broken lines represent hydrogen bonds.

Explain how hydrogen bonds are formed between ethanoic acid molecules.
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(3)
(Total 7 marks)

30.	(a)	State what is meant by the term weak acid and give one example.
Weak acid....................................................................................................................
Example.......................................................................................................................
(2) 
(b)	Write an expression for the dissociation constant, K_a, of the weak acid HA and state the units of K_a.
Expression...................................................................................................................
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Units............................................................................................................................
(2) 
(c)	When water is cooled, the pH increases but the water remains neutral.
(i)	Explain why the pH increases.
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(ii)	Explain why water remains neutral.
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(4) 
 (Total 8 marks)

31.	Solution W is 0.16 M HX, a weak monoprotic acid. It has a pH of 2.74.
(a)	(i)	Explain the terms weak and strong as applied to acids or bases.
(ii)	Determine a value of the acid dissociation constant of the weak acid HX using the expression
	
	where c is the original concentration of HX. Explain why it is reasonable to use this approximation.
(6)
 (Total 6 marks)

32.	This question concerns the weak acid, ethanoic acid, for which the acid dissociation constant, K_a, has a value of 1.74 × 10^–5mol dm^–3 at 25°C.

	In each of the calculations below, give your answer to 2 decimal places.
(a)	Write an expression for the term pH. Calculate the pH of a 0.150 mol dm^–3 solution of ethanoic acid.
(4)
 (Total 4 marks)