Topic 9 Acids and Bases 9.1

pH and Properties of Strong and Weak Acids and Bases SL/HL

1. Solutions of 0.1 mol dm -3 sodium hydroxide and 0.1 mol dm -3 ammonia have different electrical conductivities.

(i) State and explain which solution has the greater conductivity. [1]

(ii) The pH value of 0.1 mol dm-3 ammonia solution is approximately 11. State andexplain how the pH value of the 0.1 mol dm-3 sodium hydroxide solution wouldcompare.

[2]

2. (a) By means of balanced equations, give three different types of chemical reaction of an acid,such as aqueous sulphuric acid. [3]

(b) What is the difference between a strong acid and a weak acid? How could you distinguishbetween them experimentally? [4]

(c) A 0.01 mol dm -3 solution of hydrochloric acid has a pH value of 2. Suggest, with a reason, the pH values of

(a) 0.10 mol dm-3 hydrochloric acid. [2]

(b) 0.10 mol dm -3 ethanoic acid. [2]

(c) Two acidic solutions, A and B, of equal concentration, have pH values of 2 and 6respectively.

(i) Indicate which acid is stronger and calculate how many times more acidic it is. [3]

(ii) Give two ways in which solution A could be treated to produce a solution of pH 6. [2]

3. (a) Give examples of both a strong base and a weak base, clearly indicating which is which. [2]

(b) Give an equation for the reaction between carbonic acid and one of the bases given in (e). [2]

(c) Carbonic acid can be used to treat wasp (an insect) stings.

(i) Suggest what this indicates about the nature of wasp stings.

(ii) Name the type of reaction that occurs.

(iii) Explain why hydrochloric acid is not used to treat wasp stings. [3]

(d) Ammonia acts as a base in water. Write a balanced equation for this reaction and statewhat would be observed if the final solution were tested with pH paper. [3]

Topic 9 Acids and Bases pH, Buffers and simple titrations 9.2 SL/HL

1. Solutions P, Q, R and S have the following properties:

P :pH = 8 Q:[H+] = 1x10-3 moldm-3 R: pH = 5 S: :[H+] = 2x10-7 moldm-3

When these solutions are arranged in order of increasing acidity (least acidic first), the correct order is

A. P,S,R,Q.

B. Q,R,S,P.

C. S,R,P,Q.

D. R,P,Q,S.

2. A buffer solution can be prepared by adding which of the following to 50 cm3 of 0.10 mol dm-3

CH3COOH(aq)?

I. 50cm3of 0.10 moldm-3 CH3COONa(aq)

II. 25cm3of 0.10 moldm-3 NaOH(aq)

III. 50cm3of 0.10 moldm-3 NaOH(aq)

A. I only

B. I and II only

C. II and III only

D. I, II and III

3. 25.0 cm3 of hydrochloric acid of known concentration is titrated with a dilute sodium hydroxide solution. The pH of the mixture is measured continuously as shown in the graph below:

(a) (i) From the graph, determine the pH after 10.0 cm3 of sodium hydroxide solution isadded. [1]

(ii) Determine the concentration of hydrochloric acid before titration and state its units. [2]

(iii) from the graph, determine the volume of sodium hydroxide solution required toneutralise the hydrochloric acid. [1]

(iv) Calculate the concentration of the sodium hydroxide solution and state its units. [I]

(b) (i) Hydrochloric acid is a strong acid, whereas ethanoic acid is a weak acid. What is thedifference between a strong acid and a weak acid? [1]

(ii) What mass of ethanoic acid would you use and how would you prepare 0.500 dm3 of a0.500 moldm-3 ethanoic acid solution? (Mr of ethanoic acid = 60.0) [2]

4. Carbonic acid (H2C03) is described as a weak acid and hydrochloric acid (HCl) is described as a strong acid.

(a) Explain, with the help of equations, what is meant by strong and weak acid using the above acids as examples.

[4]

(a) Outline two ways, other than using pH, in which you could distinguish between carbonic acidand hydrochloric acid of the same concentration. [4]

(c) A solution of hydrochloric acid, HCl(aq), has a pH of 1 and a solution of carbonic acid,H2CO3(aq), has a pH of 5. Determine the ratio of the hydrogen ion concentrations in thesesolutions.

[2]

(d) 30 cm3 of 0.100 moldm-3 CH3COOH is placed in a beaker and mixed with 10 cm3 of 0.100 moldm-3 NaOH.

Explain, with the help of an equation, how the solution formed acts as a buffer Solutionwhen a small quantity of acid is added to it. [3]

5. (a) State what is meant by the term buffer solution. [2]

(b) State and explain whether each of the following solutions will form a buffer solution

(i) A 1.0 dm3 solution containing 0.10 mol NH3 and 0.20 mol HCl [2]

(ii) A 1.0 dm3 solution containing 0.20 mol NH3 and 0.10 mol HCl [2]

Topic 9 Acids and Bases 9.3

Brønsted Lowry Acids and Bases and Lewis Theory HL ONLY

1. A Brønsted-Lowry base is defined as a substance which

A. accepts H+ ions.

B. produces OH- ions.

C conducts electricity.

D. donates protons.

2. NH3(aq}+HNO2(aq)→NH4+(aq) + NO2

-(aq)

For this reaction, a Brønsted-Lowry acid is

A. NH3(aq) because it contains the largest number of hydrogen atoms.

B. NH3 (aq) because it accepts a proton from HNO2 (aq).

C. HNO2(aq) because it has lone pairs of electrons on the oxygen atoms.

D. HNO2(aq) because it donates a proton to NH3(aq).

3. Which of the following represents a conjugate acid-base pair in this reaction?

CH3COOH(aq)+H2O(1) ↔ CH3COO-(aq) + H3O+ (aq)

A. CH3COOH/H2O

B. CH3COOH/ CH3COO-

C. CH3COOH/H3O+

D. CH3COO-/H3O+

4. Which equation represents an acid-base reaction according to the Lewis theory but not according to the Brønsted-Lowry theory?

A. CO3 2-(aq) + 2H+(aq) → H2O(l) + CO2(g)

B. Cu2+(aq) + 4NH3(aq) → Cu(NH3)42+(aq)

C. BaO(s) + H2O(l) → Ba2+(aq) + 2OH- (aq)

D. NH3(g) + HCl(g) → NH4Cl(s)

5. In aqueous solution, hydrochloric acid is a strong acid and ethanoic acid is a weak acid.

(a) Use the Brønsted-Lowry theory to state why both substances are classified as acids. [I]

. Ethanoic acid, CH3COOH, is a weak acid.

(b) What is meant by the term weak acid? [I]

(c) Give the equation for the reaction of ethanoic acid with water and clearly identify all theBrønsted-Lowry acids and bases. [3]

(d) Give the structural formula for the conjugate base of ethanoic acid. [1]

(e) In the following reactions identify clearly the acid, conjugate base, base and conjugate acid;

(i) HNO3 + H2SO4↔H2NO3++ HSO4

- [1]

(ii) CH3CH2NH2 + H2O↔ OH- +CH3CH2NH3+ [1]

(f) Using the equation (i) in (e), state and explain the relative strengths of nitric and sulphuricacid. [2]

(g) 2H2O(1) → H3O+ (aq) + OH-(aq)

use the Brønsted-Lowry Theory to discuss the acidic and/or basic nature of water. [2]

6. In aqueous solution, sodium hydroxide is a strong base and ammonia is a weak base.

(a) Use the Brønsted-Lowry theory to state why both substances are classified as bases. [1]

Sodium hydrogencarbonate dissolves in water forming an alkaline solution according to the following ionic equilibrium:

HCO3- (aq) + H2O(1) → H2CO3 (aq) + OH- (aq)

(b) Why is the solution alkaline? [2]

(c) Using the Brensted-Lowry theory, state, with a brief explanation, whether the HCO3 ion isbehaving as an acid or as a base. [2]

(d) Identify the conjugate base of carbonic acid, H2CO3. [1]

(e) Write an equation to show the reaction of ammonia with water and classify each product as aBrønsted-Lowry acid or base. [2]

(f) What is the conjugate base of the hydroxide ion, OH-? [1]

Topic 9 Acids and Bases 9.4

Calculations involving Acids and Bases HL ONLY

1. The Ka for a weak monoprotic acid is 1 x 10-5 mol dm-3. What will be the pH of a solution of this acid with a concentration of 0.1 mol dm-3 ?

A. 1

B. 3

C. 4

D. 6

2. What is the concentration of OH- ions (in mol dm-3) in an aqueous solution in which [H+] = 2.0xl0-3 mol dm-3? (Kw =1.0xl0-14mol 2dm-6)

A. 2.0xl0-3

B. 4.0xl0-6

C. 5.0xl0-12

D. 2.0xl0-17

3. A 0.1 mol dm-3 solution of a weak acid has a pH = 3.0. What is Ka for this acid?

A. 1.0xl0-1

B. 1.0xl0-3

C. 1.0xl0-5

D. 1.0xl0-6

4. What is the relationship between Ka and pKa ?

5. The pH value of a 1.00x10 -3 mol dm-3 solution of sodium hydroxide is

A. 3.

B. 8.

C. 11.

D. 14.

6. (a) Write an equation to show the ionisation of propanoic acid in water. [1]

(b) Give the equilibrium expression for this reaction. [1]

(c) Using information from Table 16 in the Data Booklet, determine the pH of a 0.200 mol dm -3

aqueous solution of propanoic acid. State the approximation that you have made in arrivingat your answer. [3]

7. The following table is to be used as appropriate in answering this question.

Solution Solutes Conc, (mol dm-3) K (mol dm-3)A HCl(aq) 0.25

B CH3COOH(aq) 0.25 Ka = 1.8 x 10-5

C NaOH(aq) 0.25D NH3(aq) 0.25 Kb= 1.8 x 10-5

E CH3COONa(aq) 0.25

(a) Calculate the pH of solution A and indicate why no Ka value is given forthis solution. (2 marks]

(b) Calculate the pH of solution B. State any assumptions made. [4 marks]

(c) Use information from Table 16 of the Data Booklet to calculate the value of the ionisationconstant, Ka, of ethanoic acid. [1]

(d) Write the expression for the ionisation constant, Ka of ethanoic acid. [1]

(e) Use your answers to (c) and (d) to calculate the pH value of a 0.050 mol dm -3 solution ofethanoic acid. [2]

Topic 9 Acids and Bases 9.5

Buffer Calculations HLONLY

1. Which of the following combinations produce a buffer solution when equal volumes are mixed?

I. O.lM HCl and O.lM NH4C1

II. 0.1M HCl and 0.2M NH3

III. 0.1M NH3 and O.lM NH4C1

A. I only

B. III only

C. II and III only

D. I, II and II

2. A certain buffer solution contains equal concentrations of X (aq) and HX(aq). The Kb value for X-(aq) is 1.0 x 10-10. What is the pH of the buffer?

A. 1

B. 4

C. 5

D. 10

3. What mass of sodium propanoate, Na+CH3CH2COO- , is required in 500 cm3 of a solution of 0.200 moldm-3 propanoic acid to give a pH of 4.87? [3]

4. 30cm3 of 0.100 moldm-3 CH3COOH is placed in a beaker and mixed with 10 cm of0.100 moldm-3 NaOH

Calculate the pH of the buffer solution. (Ka of CH3COOH = 1.74xl0-5 moldm-3) [4].

5. A mixture of benzoic acid and sodium benzoate can act as a buffer solution.

(i) Define the term buffer solution and describe what happens when acid is added to a buffer solution. [5]

(ii) Calculate the pH of a solution containing 7.2 g of sodium benzoate in 1.0 dm3 of

2.0x10-2 mol dm-3 benzoic acid. (Ka=6.3x10-5 mol dm-3) stating any assumptions that you have made. [6]

6. In a titration experiment, 40.0 cm3 of 0.150 mol dm -3 NaOH was added to 60.0 cm3 of0.200 mol dm-3 CH3(CH2)3COOH (Ka = 1.38x 10 -5 mol dm-3). Calculate the pH of thismixture. [4]

Topic 9 Acids and Bases

Salt Hydrolysis, Titrations and Indicators

1. Which salt would form a neutral solution when dissolved in water?

A. FeCl3

B. Na2CO3

C. KBr

B. NH4NO3

9.

6HLONL

Y

2. Which curve is produced by the titration of a 0.1 mol dm3 weak base with 0.1 mol dm'3 strong acid?

3. Sodium hydroxide solution is added to aqueous hydrochloric acid. The graph of pH against volume of sodium hydroxide solution added is shown below:

Sketch clearly labelled corresponding graphs for each of the following and suggest a suitable indicator in each case:

(i) The addition of sodium hydroxide solution to aqueous ethanoic acid. [3]

(ii) The addition of ammonia solution to aqueous hydrochloric acid. [3]

4. (a) Explain why an aqueous sodium ethanoate solution is basic whereas an aqueous ammoniumethanoate solution is approximately neutral.

[4]

(b) If the pH of water in a swimming pool goes above 8, aluminium sulfate, Al2(SO4)3, is addedto it to adjust its pH. With the help of formulas and acid—base properties of the ions present,explain how this is achieved.

[3]

4. Benzoic acid is a weak monoprotic acid.

(a) Explain the term monoprotic acid. [1]

(b) The experimentally determined graph below shows the change in pH when 0.10 mol dm-3 aqueous sodium hydroxide is added to 25 cm3 of 0.10 mol dm-3 aqueous benzoic acid.

(i) Calculate the pH when the benzoic acid is half-neutralised and explain how you arrivedat your answer. [2]

(ii) Explain by reference to Table 17 of the Data Booklet why phenolphthalein is a suitableindicator for this titration. [2]

(c) Explain how an indicator works using

HIn → H+ + In-

where HIn represents the formula of the indicator. [2]

(d) The experiment was repeated using 25 cm3 of 0.10 mol dm hydrochloric acid instead of benzoic acid.

(i) Sketch the graph you would expect from the results of this second experiment. [3]

(ii) State and explain any similarities and differences between the two graphs. [4]

5. The indicator bromophenol blue, HIn (aq) has a form that is yellow and an In -(aq) form that is blue.

(a) Write an equation to show how bromophenol blue acts as an indicator. [1]

(b) State and explain the colour of bromophenol blue [3]

(i) on the addition of a strong acid.

(ii) at the equivalence point of a titration.