ACIDS AND BASES

INSTRUCTIONS FOR QUESTION 1

  • Answer this question on the ANSWER SHEET provided. Write your nameon the answer sheet.
  • Four possible answers, indicated by A, B, C and D, are supplied with each question. Each question has only one correct answer. Choose only that answer, which in your opinion, is the correct or best one and mark the appropriate block on the answer sheet with a cross (X).
  • Do not make any other marks on the answer sheet. If more than one block is marked, no marks will be awarded for that answer.

QUESTION 1

1.1H3PO4 is dissolved in H2O and after some time an equilibrium is established. The Lowry-Brönsted bases present in this reversible reaction are …

AH3PO4 and H2PO4–

BH2O and OH–

CH2O and H2PO4–

DH3PO4 and H3O+(4)

1.2The addition of which one of the following substances to water will increase the pH of the solution?

ASodium ethanoate

BSodium sulphate

CAmmonium chloride

DSodium chloride(4)

1.3When sodium hypochlorite (NaOCℓ) dissolves in water, the pH of the solution is 9,82. Which one of the following equations offers the best explanation for this?

A2H2O H3O+ + OH–

BNaOCℓ Na+ + OCℓ–

COCℓ– + H2O HOCℓ + OH–

DNaOCℓ + H2O NaCℓ + O2 + H2(4)

1.4HPO42– can act as an ampholyte. In which one of the following reactions does HPO42– act as a Lowry-Brönsted acid?

AHPO42– + H+H2PO4–

BHPO42– + HPO42– 2HPO42–

CHPO42– + H2O H2PO4– + OH–

DHPO42– + H2O PO43– + H3O+(4)

1.5Consider the incomplete reaction equation below:

CH3CO2H(aq) / + / C5H5N(aq) / / ______/ + / C5H5NH+ (aq)

The conjugate base of CH3CO2H is …

ACH3CO2–

BCH3CO2+

CC5H6N

DC5H5NH+(4)

1.6Which ONE of the following 0,1 mol.dm–3 solutions has the lowest pH?

AHCℓ

BH2SO4

CNaOH

DCH3COOH(4)

1.7H2S can be prepared via the reaction of HCℓ and FeS, according to the following equation:

FeS (s) / + / 2HCℓ (aq) /  / FeCℓ2 (aq) / + / H2S (g)

In this reaction …

AFeS acts as a base.

BFeS acts as an oxidising agent.

CHCℓ acts as a reducing agent.

DHCℓ is the conjugate acid of FeS.(4)

1.8The following equation represents the ionization of water:

H2O(ℓ) / / H+ (aq) / + / OH–(aq)

When the temperature of water is decreased from 298 K to 278 K, the pH increases from 7,0 to 7,4. Which one of the following statements is correct?

A[ H+ ] > [ OH–] at 278 K.

BKW = 10–14 at 278 K.

CThe forward reaction is endothermic.

DAt 278 K the rate of the forward reaction is higher than the rate of the reverse reaction. (4)

1.90,1 mol.dm–3 solutions of four substances are prepared. Which one will have the lowest pH?

ANH4NO3

BNH4OH

CKNO3

DKOH(4)

1.10Concentrated sulphuric acid is added to potassium nitrate, and a reaction takes place according to the following equation:

H2SO4 (ℓ) / + / KNO3 (s) / / KHSO4 (s) / + / HNO3 (g)

Which one of the following is a conjugate acidbase pair in this reaction?

AH2SO4 and SO42–

BHNO3 and NO3–

CH2SO4 and KNO3

DHSO4– and NO3–(4)

1.11Which one of the following ions is an ampholyte?

ANH4+

BSO42–

CCO32–

DHCO3–(4)

1.12Consider a solution of ammonium chloride (NH4Cℓ) in water.

Which one of the following statements is FALSE?

A[H+ ] < [OH– ]

B[H+ ][OH– ] = 10–14

C[H+ ] > 10–7mol.dm–3

DThe pH of the solution is less than 7.(4)

1.13 In the reaction,

X / + / H2O / / H3O+ / + / HSO4–

X represents the following:

AThe acid SO42–

BThe base SO42–

CThe acid H2SO4

DThe base H2SO4(4)

1.14The following table gives the pH values at which the colour change in three acidbase indicators occur.

Indicator / pH value
Congo red / 2,8 - 4,8
Cresol red / 7,0 - 9,1
Chlorophenol red / 5,2 - 6,8

The indicator best suited for use in the titration of ethanoic acid against a solution of sodium hydroxide, is …

ACongo red.

BCresol red.

CChlorophenol red.

Dany one of the three.(4)

1.15When a dilute solution of ethanoic acid (CH3COOH) is prepared in water, approximately 1% of the acid molecules ionise. When an amount of concentrated hydrochloric acid (HCℓ) is added to the solution, it is found that only 0,02% of the ethanoic acid molecules ionise.

The difference in the degree of ionisation can best be attributed to …

Athe common ion effect.

Bacetic acid being a weak acid.

Cthe precipitation reaction that takes place.

DCH3COOH and HCℓ forming a conjugate acidbase pair.(4)

1.16An example of a polyprotic acid is …

AHNO3

BHCℓ

CH2SO4

DCH3COOH(4)

1.17The pH of a solution of sodium hydroxide is 13. How will the [OH–] and the pH of the solution be affected if distilled water is added to the solution?

[OH–] / pH
A / Decreases / Increases
B / Decreases / Decreases
C / Unchanged / Decreases
D / Increases / Decreases

(4)

[68]

QUESTION 2

Two beakers, X and Y, contain the following solutions:

X:250 cm3 NaNO3 with a concentration of 0,5 mol.dm–3

Y:200 cm3 Cu(NO3)2 with a concentration of 0,25 mol.dm–3

The contents of the two beakers are mixed. Assume that both compounds dissociate completely in an aqueous medium. What is the concentration of the NO3– ions in the mixture? [6]

QUESTION 3

20 cm3 of a 0,2 mol.dm–3 NaOH solution is titrated against 25 cm3 of a H2SO4 solution. The balanced equation for the reaction is:

2NaOH(aq) / + / H2SO4 (aq) /  / Na2SO4 (aq) / + / 2H2O(ℓ)

3.1Calculate the concentration of the H2SO4 solution.(4)

3.2Write down the NAME of the proton donor in this reaction.(2)

[6]

QUESTION 4

When 0,05 moles of HBr(ℓ) are added to 0,005 moles of KOH(aq) in a beaker, the total volume of the solution is 75 cm3. HBr dissociates completely when dissolved in water. Calculate the pH of the final solution. [7]

QUESTION 5

6,2 g of KOH is dissolved in 250 cm3 of water. Calculate the pH of this solution.[7]

QUESTION 6

A solution of Na2CO3 is made in water, and it is found that the concentration of the hydroxide ions present in the water is increased.

6.1Write down a balanced equation for the reaction that is responsible for the increase in the concentration of the hydroxide ions. (3)

6.2Write down the formula of an ampholyte present in the reaction in 6.1.(2)

6.3When CaCℓ2 is added to the solution a precipitate of CaCO3 forms. How will this affect the pH? (Write down only INCREASES, DECREASES or STAYS THESAME.) (1)

6.4Give an explanation of the answer to 6.3 by referring to the effect that the addition of CaCℓ2has on the equilibrium of the reaction in 6.1. (3)

[9]

QUESTION 7

70 cm3 of a solution of sodium hydroxide (concentration = 0,18 mol.dm–3) reacts completely with 30 cm3 of a solution of sulphuric acid. The equation for the reaction is:

H2SO4 (aq) / + / 2NaOH(aq) /  / Na2SO4 (aq) / + / 2H2O(ℓ)

7.1Write down the FORMULA of the conjugate acid of sodium hydroxide.(2)

7.2Calculate the concentration of the solution of sulphuric acid.(4)

7.3Calculate the mass of sodium hydroxide used in the titration.(4)

[10]

QUESTION 8

An aqueous solution of ammonia is investigated.

8.1State whether the pH of this solution will be GREATER THAN 7, EQUAL TO 7 or LESS THAN 7. (2)

8.2Give a reason for your answer to 8.1.(2)

8.3Write down a suitable balanced reactionequation to substantiate your answer to 8.2.(2)

8.4If a few crystals of ammonium chloride are added to the ammonia solution, will the pH of the solution INCREASE, DECREASE or REMAIN THE SAME? (2)

8.5Supply a reason for your answer to 8.4.(2)

[10]

QUESTION 9

Washing soda contains sodium carbonate (Na2CO3). A sample of 5,13 g of washing soda crystals was dissolved in water to prepare a 250 cm3 solution. 25 cm3 of this solution is neutralised by 36 cm3 of a 0,05 mol.dm–3 sulphuric acid solution.

The balanced equation for the reaction is:

Na2CO3 (aq) / + / H2SO4 (aq) /  / Na2SO4 (aq) / + / CO2 (g) / + / H2O (ℓ)

9.1Calculate the number of moles of sulphuric acid that reacted with the sodium carbonate.(2)

9.2Calculate the mass of sodium carbonate in the sample of washing soda.(7)

9.3What percentage of the washing soda sample is sodium carbonate?(2)

[11]

QUESTION 10

Concentrated sulphuric acid (H2SO4) is added to sodium chloride crystals (NaCℓ) in test tube X. A gas develops, according to the reaction:

H2SO4 / + / NaCℓ /  / NaHSO4 / + / HCℓ
/

10.1Write down the FORMULAE of two conjugate acidbase pairs that appear in the above reaction equation. Write the acid first followed by the conjugate base, for each pair. (4)

The gas is now bubbled through test tube S, containing a saturated solution of NaCℓ.

10.2What is observed when the gas is bubbled through test tube S?(2)

10.3Name of the effect (chemical phenomenon) responsible for the observation in 10.2.(2)

The gas is now bubbled through test tube T, which contains a solution of bromothymol blue, to which a few drops of sodium hydroxide solution (NaOH) have been added. The colour of bromothymol blue at different pH values is tabled below.

pH / Colour
Smaller than 6 / yellow
6,5 - 7,5 / green
Larger than 8 / blue
/

10.4What change (if any) in colour is observed in test tube T when the gas is bubbled through the solution in test tube T? (2)

10.5After the gas had been bubbled through test tube T for some time, the reaction in test tube X stops. It is now observed that the solution in T flows rapidly up the deliverytube into X due to a drop in pressure. Give a reason for the drop in pressure. (2)

[12]

QUESTION 11

A learner determined the pH of a number of solutions. The following results were obtained:

Solution / pH
Hydrochloric acid / 1
Grape juice / 3,1
Sodium hydroxide / 13

11.1Which solution contains the highest concentration of hydrogen ions?(2)

11.2How will the pH of grape juice change when …

(Write down only INCREASES, DECREASES or STAYS THE SAME)

11.2.1distilled water is added to it?(2)

11.2.2some of the sodium hydroxide solution is added to it?(2)

11.2.3some of the hydrochloric acid solution is added to it?(2)

11.3The learner now analyses sea water, and finds the concentration of the hydroxide ions [OH–] in sea water to be 10–6mol.dm–3. Calculate the pH of sea water. (5)

[13]

QUESTION 12

A volume of 0,60 dm3 of a 0,20 mol.dm–3 NaOH solution is added to a beaker containing 0,40 dm3 of a H2SO4 solution. It was found that the pH of the final solution is 12,5 at a temperature of 25°C.

12.1Calculate the hydroxide ion (OH–) concentration on completion of the reaction.(5)

12.2Write down the balanced equation for the reaction that takes place.(3)

12.3Calculate the concentration of the initial H2SO4 solution.(7)

[15]

QUESTION 13

A learner prepares a potassium hydroxide (KOH) solution to titrate it against a sulphuric acid solution.

13.1Calculate the mass of potassium hydroxide (KOH) needed to prepare 400 cm3 of a 0,15 mol.dm–3 KOH solution. (5)

13.225 cm3 of the above solution is used to neutralise 30 cm3 of a sulphuric acid solution. Determine the concentration of the sulphuric acid solution. (5)

13.3When only one drop of the sulphuric acid solution still had to be added to reach the equivalence (end) point of the titration, the concentration of the OH– in the mixture was 1,23 x 10–4 mol. dm–3. Determine the pH of the solution at this specific point. (6)

[16]

QUESTION 14

A 200 cm3 solution of hydrochloric acid has a pH of 1.

14.1Calculate the volume of water that must be added to this solution to change the pH to 2.(5)

Sodium carbonate crystals (Na2CO3.10H2O) are used to neutralise the original hydrochloric acid (with a pH of 1).

14.2Write down the balanced equation for the neutralisation reaction.(3)

14.3Calculate the mass of sodium carbonate crystals that will be required to neutralise 200 cm3 of the hydrochloric acid solution. (6)

14.4Choose from the following table the most suitable indicator for the reaction.(1)

Indicator / pH range in which the colour changes
Methyl red / 4,8 - 6,0
Neutral red / 6,8 - 8,0
Chlorophenol red / 7,0 - 8,8

14.5Give a reason for the answer to question 14.4.(2)

[17]

QUESTION 15

The acid HF ionises according to the following equation:

HF(aq) / / H+ (aq) / + / F– (aq)

When a 0,10 mol.dm–3 solution of HF is prepared, it is found that the concentration of the F– (aq) ions is 0,018 mol.dm–3. The temperature of the solution is 25°C.

15.1Is HF a strong acid?(1)

15.2Give an explanation for the answer to 15.1.(2)

15.3How will the pH of the solution change when solid NaF is dissolved in the solution? (Write down only DECREASES, INCREASES or STAYS THE SAME) (2)

15.4Give an explanation for your answer to 15.3.(3)

0,50 dm3 of the 0,10 mol.dm–3 HF solution is now added to 0,80 dm3 of a 0,25 mol.dm–3 solution of NaOH.

15.5 Calculate the number of moles of hydroxide ions (OH–) in the solution at the completion of this reaction. (6)

15.6Calculate the pH of the solution at the completion of the reaction.(6)

[20]

QUESTION 16

16.1Define a base according to the LowryBrönsted model.(2)

In preparing a standard basic solution, a learner dissolves 14g of potassium hydroxide (KOH) in 1,0 dm3of water.

16.2What is a standard solution?(2)

16.3Calculate the pH of the KOH solution.(9)

In a titration, 30 cm3 of the KOH solution prepared in question 16.2 is neutralised by a sulphuric acid (H2SO4) solution with a concentration of 0,10 mol.dm–3.

16.4Write down the balanced equation of the neutralisation reaction that takes place.(3)

16.5Calculate the volume of sulphuric acid used in the titration.(5)

16.6What is the pH of the mixture at the equivalence (end) point of the titration? (Write only LESS THAN SEVEN, MORE THAN SEVEN or EQUAL TO SEVEN as your answer.) (2)

[23]

[TOTAL: 250]

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