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Quiz 6 of 12

# 2017 Chemistry WAEC Theory Past Questions

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## Question 1

(a)(i) What is an acid-base indicator?

Answer: An acid-base indicator is either a weak acid or weak base which gives different colours in an acid/base medium. It is also a dye (organic) which changes colour according to the PH/hydrogen ion concentration of the solution.

(ii) Give one example of an acid-base indicator.

• Phenolphalein
• Methyl orange
• Methy red
• Litmus paper
• Methyl violet
• Methyl yellow
• Bromothamol blue
• Bromocresol green

(b) State the property exhibited by nitrogen(IV) oxide in each of the following equations

i.  4Cu  +  2NO2 → 4CuO  +  N2

Answer: NO2 is an oxidizing agent

ii. H2O  +  2NO2  → HNO3  +  HNO2

Answer: NO2 is an oxidizing agent as well as a reducing agent.

(c)(i) Define enthalpy of combustion.

Answer: Enthalpy of combustion is the amount of heat evolved when one mole of a substance is burnt completely in excess oxygen.

(ii) State why the enthalpy of combustion is always negative.

Answer: Enthalpy of combustion is always negative because heat is always given out. The action is exothermic.

(d)(i) Distinguish between a primary cell and a secondary cell.

• Primary cells are not rechargeable, while a secondary cell can be recharged
• Primary cells cannot supply a steady current, while secondary cells supply a steady current.
• Primary cells do not last long, while secondary cells last long
• Reactions in primary cells are not reversible, while reactions in secondary cells are reversible.

(ii) Give an example of each of the cells stated in I (d)(i).

Examples of primary cells

• Dry cell
• Daniel cell
• Alkaline battery
• Leclanche battery

Examples of secondary cells

• Lithum-ion battery
• Car battery

(e) Define the term mole.

Answer: A mole is the amount of substance containing 6.02 x 1023 particles of that substance. It is the amount of substance in 12g of carbon -12.

(f) Calculate the amount of hydrochloric acid in 40.0cm3 of 0.40 moldm-3 dilute HCl

Solution

Given : substance : HCl

volume of solution = 40.0cm3

molar concentration = 0.40 moldm-3

Find: Amount of HCl (in mole)

Amount of HCl in mole = $$\frac{molar \: concentration \: \times \: volume \: of \: solution}{1dm^3 = 1000cm^3}$$

= $$\frac{0.4 \: \times \: 40}{1000}$$

= 0.16mol

(g) Name two substances which can be used as electrodes during the electrolysis of acidified water.

• Carbon or graphites
• Platinum

(h) List two forces of attraction that can exist between covalent molecules.

• Van der Waal’s forces or induced dipole-dipole forces
• Hydrogen bonds
• {Permanent} dipole-dipole interactions or dipolar forces

(i) Name the products formed when butane undergoes incomplete combustion.

• Carbon (II) oxide
• Water

(j) Write the electronic configuration of 26F3+

Answer: 1 S2 2 S2 P6 3 S2 3P6 3d5

## Question 2

(a)(i)

I. State the periodic law.

Answer: The periodic law states that the properties of elements are a periodic function of their atomic numbers.

II. What is meant by the term periodic property of elements?

Answer: Periodic property is any property {Physical or Chemical} of an element which changes regularly or shows variation with increasing atomic number.

(ii) List three properties of an element which show periodicity.

• Melting or boiling point
• Ionization energy
• Electronegativity/Electropositivity
• Electrical and thermal conductivity

(iii) Explain briefly how each of the properties listed in  2a(ii)  varies across the period.

• Melting point or boiling point of metallic elements increases due to an increase in the strength of the metallic bond.
• Melting or boiling point of non-metallic elements decreases due to a decrease in Van der Waal’s forces of attraction.
• Ionization energy increases due to an increase in proton number or increase in nuclear charge or decreased atomic radius
• Electronegativity of elements increases due to increased nuclear charge/decreased atomic radius
• Atomic radius decreases due to increased nuclear charge or increase in proton number
• Metallic character {electrical or thermal} decreases owing to an increase in covalent character.

(b) Defulle relative atomic mass.

Answer: This is the ratio of the average mass of one atom of an element on a scale in which the atom of one atom carbon-12 weighs a.m.u. It is also the number of times the average mass of one atom of an element is heavier than one twelfth (1/12) the mass of one atom of carbon -12.

(c)(i) What phenomenon is exhibited by an element Z which exist as $$\scriptsize ^{35}_{17}Z$$and $$\scriptsize ^{37}_{17}X$$

(ii) What accounts for the difference in the mass numbers of the element Z?

Answer: difference in the number of neutrons

(iii) Calculate the relative atomic mass of Z if the percentage abundance of $$\scriptsize ^{37}_{17}Z$$ is 75%

Solution:

Percentage abundance of $$\scriptsize ^{37}_{17}Z$$ = 100 - %abundance of $$\scriptsize ^{35}_{17}Z$$

= 100 – 75 = 25%

Relative atomic mass = $$\left(\frac{75}{100} \scriptsize \: \times \: 35 \right) \: + \: \left(\frac{25}{100} \scriptsize \: \times \: 37 \right)$$

= 26.25 + 9.25 = 35.5

(d)(i) State the method used for collecting each of the following gases:

I. CO2

Answer:  CO2 – downward delivery / upward displacement

II. HCI

Answer:  HCI – downward delivery / upward displacement of air.

III. H2

Answer:  H2– Upward delivery / downward displacement of air.

(ii) Give a reason for your answer stated in (d)(i) I and II

I. CO2  – heavier / denser than air

II. HC – heavier / denser than air

## Question 3

(a) Consider the following reaction equation:

$$\scriptsize C_{12}H_{26} \: \underset{350^oC}{\overset{Al_2O_3}{\rightarrow}} \: X \: + \: C_{8}H_{18}$$

(i) What type of reaction is represented by the equation?

(ii) Write the molecular formula of X.

(iii) Draw the structure of two isomers of X.

(iv) Name the isomers drawn in (a)(iii).

1.          But – 1 – ene
2.           But – 2 – ene
3.           2 – methylpropene

(v) Write a balanced equation for the reaction between X and hydrogen.

Answer:  C4H8  +  H2  → C4H10

(b) Describe one test for fats.

Answer: place a small amount of the substance on a filter paper and melt/warm/rub. The formation of a translucent spot on the filter paper indicates the presence of fats

OR

Treat with sudan III reagent. Add sudan III to fat. A red colour is observed.

(c) Sulphur (IV) oxide is converted to tetraoxosulphate (VI) acid according to the following equation:

2SO2(g)   +  O2(g)   +  2H2O(l)   →   2H2SO4(aq)

If 1.5 moles of oxygen reacts with sulphur (IV) oxide, calculate the mass of tetraoxosulphate (VI) acid produced.

[H = 1.0; O = 16.0; S = 32.0].

Solution

Molar mass of H2SO4 = (1 x 2 + 32 + 16)

= 98gmol-1

From the equation

1 mole of O2 forms 2 moles of H2SO4

1.5moles of oxygen will produce 3 moles of H2SO4

Mass of 3 moles of H2SO4 = 3 x 98

= 294g

Or

1 mole O2 →  2 x 98g  H2SO4

1.5 mole O2   →  1.5 x 2 x 98

= 294g

(d) Consider the following neutralization reactions:

CH3 COOH  +  NaOH → CH3 COONa  + H2O;  ∆H1

CH3 COOH  + NH4 OH → CH3 COONH4  +  H2O; ∆H2

NaOH + HCI   →   NaCl  + H2O; ∆H3

(i) Arrange the enthalpy changes for the reactions in order of increasing magnitude.

Answer: ∆H2   <     ∆H1      <  ∆H3

(ii) Explain briefly your order in (d)(i).

Answer:  Naturalization involves the combination of H+ and OH- ions to give a specific amount of energy. The greater the ionization the greater the energy given off.

∆H2 involves a weak acid and a weak base

∆H1 involves a strong base and a weak acid

∆H3 involves a strong acid and a strong base

(e) Consider the following substances.

Cu(s) , BeCl2(s), NaH(s), HF(s) and CCl4(l)

State the substance(s) which;

(i) can conduct electricity

(ii) is/are soluble in water.

Answer:  NaH(g) , BeCl2(s), HF(l) are soluble in water.

## Question 4

(a)(i) Describe briefly the industrial preparation of ammonia.

Answer: Dry nitrogen (N2(g)) and dry hydrogen (H2(g))are mixed together in volume proportion if 1:3 respectively. The reaction takes place over a pressure of 200 - 500 atm with finely divided iron as a catalyst and at a temperature of 450°C — 500°C.

(ii) Write a balanced equation for the reaction in (a)(i).

Answer:  3H2(g)  +  N2(g)   $$\leftrightharpoons$$ 2NH3(g)

(iii) State one way of increasing the yield of ammonia in 4(a)(i).

• Continuous removal of ammonia as it is formed
• Liquefying the ammonia
• Low temperature
• High pressure

(iv) State two uses of ammonia.

• Manufacture of fertilizers
• As a precipitating agent
• Manufacture of trioxonitrale (v) acid
• Manufacture of trioxocarbonate (iv), textiles, pesticides
• Used in detergents/cleaning agent
• Manufacture of plastics
• Laundry works
• As a refrigerant

(b) Describe briefly, one chemical test for each of the following gases in the laboratory:

(i)Hydrogen

Answer:  Hydrogen – when a lighted wooden splint is brought to the tip of a test tube containing the gas, the gas gives up a ‘pop’ sound.

(ii) Carbon (IV) oxide

Answer:  Carbon (iv) oxide – Bubble gas through lime water {Ca(OH)2} solution. The lime water turns milky.

(iii) Oxygen

Answer:  Oxygen – when a glowing {wooden} splint is dipped into a test tube of the gas, the splint rekindles.

(c)(i) State the composition of water gas.

Carbon(ii) oxide {CO}

Hydrogen {H2

(ii) List two uses of water gas.

It is used as fuel

Industrial manufacture of hydrogen, method, butanol

(d) Describe briefly a simple experiment to determine the type of hardness in a sample of water.

• Boil a sample of water

If it forms lather easily, then it is temporary hardness.

If it forms lather with difficulty or does not form lather easily, then it is permanent hardness.

## Question 5

(a)(i) Describe briefly how trioxonitrate (V) ions could be tested for in the laboratory.

Answer: Add freshly prepared iron (ii) tetraoxosuplhate (vi) {FeSO4} solution followed by conc. H2SO4.  A brown ring is observed.

OR

Add concentrated H2SO4 to the substance, then heat. Brown fumes of NO2 is observed.

OR still

Mix substance with copper and con. H2SO4 and warm. A reddish-brown fume of NO2 is observed.

(ii) State two uses of each of the following compounds:

I. Sodium chloride;

• Food seasoning
• Food preservative
• Soap production
• Crude oil refining
• Water softener
• Production of Chlorine/hydrogen/sodium metal
• Used in tannery industry

II. Sodium trioxocarbonate (IV).

• Manufacture of glass/borax
• In detergents/soap
• Manufacture of paper
• Softening hard water
• Manufacture of NaOH

(b) Write balanced equations for the reactions involved in the extraction of iron in the blast furnace.

C(s)  +  O2(g) →  CO2(g)

CO2(g)  + C(s) → 2CO(g)

Fe2O3  +  3CO(g) → 2Fe  +  3CO2(g)

Fe3O4  +  4CO → 3Fe  +  4CO2

CaO   +  SiO2 → CaSiO3

CaCO3 →  CaO  +  CO2

(ii) State Faraday's first law of electrolysis.

Answer: It states that the mass of an element discharged at the electrode is directly proportional to the quantity of electricity passed.

(iii) State two applications of electrolysis.

• Electroplating of metals
• Extraction of metals
• Purification of metals
• Preparation of compounds/elements

(c) Concentrated tetraoxosulphate (VI) acid is added to sugar crystals in a beaker. State what would be observed. Explain briefly your answer.

• Beaker becomes warm/formation of black solid
• The concentrated acid dehydrates the sugar
• A mass of carbon/black soot/sugar charcoal is formed.
• Reaction is exothermic

(d) Write an equation for the reaction of zinc powder with:

(i) Dilute tetraoxosulphate (VI) acid;

Answer: Zn(s) + H2SO4(aq) → ZnSO4(aq)  +  H2(g)

(ii) Concentrated tetraoxosulphate (VI) acid.

Answer: Zn(s)  +  2H2SO4(l) → ZnSO4(aq)  +  SO2(g)  +  2H2O(l)

(e) What property of concentrated tetraoxosulphate (VI) acid is shown in (d)(ii)

## Question 1

(a)(i) What is an acid-base indicator?

Answer: An acid-base indicator is either a weak acid or weak base which gives different colours in an acid/base medium. It is also a dye (organic) which changes colour according to the PH/hydrogen ion concentration of the solution.

(ii) Give one example of an acid-base indicator.

• Phenolphalein
• Methyl orange
• Methy red
• Litmus paper
• Methyl violet
• Methyl yellow
• Bromothamol blue
• Bromocresol green

(b) State the property exhibited by nitrogen(IV) oxide in each of the following equations

i.  4Cu  +  2NO2 → 4CuO  +  N2

Answer: NO2 is an oxidizing agent

ii. H2O  +  2NO2  → HNO3  +  HNO2

Answer: NO2 is an oxidizing agent as well as a reducing agent.

(c)(i) Define enthalpy of combustion.

Answer: Enthalpy of combustion is the amount of heat evolved when one mole of a substance is burnt completely in excess oxygen.

(ii) State why the enthalpy of combustion is always negative.

Answer: Enthalpy of combustion is always negative because heat is always given out. The action is exothermic.

(d)(i) Distinguish between a primary cell and a secondary cell.

• Primary cells are not rechargeable, while a secondary cell can be recharged
• Primary cells cannot supply a steady current, while secondary cells supply a steady current.
• Primary cells do not last long, while secondary cells last long
• Reactions in primary cells are not reversible, while reactions in secondary cells are reversible.

(ii) Give an example of each of the cells stated in I (d)(i).

Examples of primary cells

• Dry cell
• Daniel cell
• Alkaline battery
• Leclanche battery

Examples of secondary cells

• Lithum-ion battery
• Car battery

(e) Define the term mole.

Answer: A mole is the amount of substance containing 6.02 x 1023 particles of that substance. It is the amount of substance in 12g of carbon -12.

(f) Calculate the amount of hydrochloric acid in 40.0cm3 of 0.40 moldm-3 dilute HCl

Solution

Given : substance : HCl

volume of solution = 40.0cm3

molar concentration = 0.40 moldm-3

Find: Amount of HCl (in mole)

Amount of HCl in mole = $$\frac{molar \: concentration \: \times \: volume \: of \: solution}{1dm^3 = 1000cm^3}$$

= $$\frac{0.4 \: \times \: 40}{1000}$$

= 0.16mol

(g) Name two substances which can be used as electrodes during the electrolysis of acidified water.

• Carbon or graphites
• Platinum

(h) List two forces of attraction that can exist between covalent molecules.

• Van der Waal’s forces or induced dipole-dipole forces
• Hydrogen bonds
• {Permanent} dipole-dipole interactions or dipolar forces

(i) Name the products formed when butane undergoes incomplete combustion.

• Carbon (II) oxide
• Water

(j) Write the electronic configuration of 26F3+

Answer: 1 S2 2 S2 P6 3 S2 3P6 3d5

## Question 2

(a)(i)

I. State the periodic law.

Answer: The periodic law states that the properties of elements are a periodic function of their atomic numbers.

II. What is meant by the term periodic property of elements?

Answer: Periodic property is any property {Physical or Chemical} of an element which changes regularly or shows variation with increasing atomic number.

(ii) List three properties of an element which show periodicity.

• Melting or boiling point
• Ionization energy
• Electronegativity/Electropositivity
• Electrical and thermal conductivity

(iii) Explain briefly how each of the properties listed in  2a(ii)  varies across the period.

• Melting point or boiling point of metallic elements increases due to an increase in the strength of the metallic bond.
• Melting or boiling point of non-metallic elements decreases due to a decrease in Van der Waal’s forces of attraction.
• Ionization energy increases due to an increase in proton number or increase in nuclear charge or decreased atomic radius
• Electronegativity of elements increases due to increased nuclear charge/decreased atomic radius
• Atomic radius decreases due to increased nuclear charge or increase in proton number
• Metallic character {electrical or thermal} decreases owing to an increase in covalent character.

(b) Defulle relative atomic mass.

Answer: This is the ratio of the average mass of one atom of an element on a scale in which the atom of one atom carbon-12 weighs a.m.u. It is also the number of times the average mass of one atom of an element is heavier than one twelfth (1/12) the mass of one atom of carbon -12.

(c)(i) What phenomenon is exhibited by an element Z which exist as $$\scriptsize ^{35}_{17}Z$$and $$\scriptsize ^{37}_{17}X$$

(ii) What accounts for the difference in the mass numbers of the element Z?

Answer: difference in the number of neutrons

(iii) Calculate the relative atomic mass of Z if the percentage abundance of $$\scriptsize ^{37}_{17}Z$$ is 75%

Solution:

Percentage abundance of $$\scriptsize ^{37}_{17}Z$$ = 100 - %abundance of $$\scriptsize ^{35}_{17}Z$$

= 100 – 75 = 25%

Relative atomic mass = $$\left(\frac{75}{100} \scriptsize \: \times \: 35 \right) \: + \: \left(\frac{25}{100} \scriptsize \: \times \: 37 \right)$$

= 26.25 + 9.25 = 35.5

(d)(i) State the method used for collecting each of the following gases:

I. CO2

Answer:  CO2 – downward delivery / upward displacement

II. HCI

Answer:  HCI – downward delivery / upward displacement of air.

III. H2

Answer:  H2– Upward delivery / downward displacement of air.

(ii) Give a reason for your answer stated in (d)(i) I and II

I. CO2  – heavier / denser than air

II. HC – heavier / denser than air

## Question 3

(a) Consider the following reaction equation:

$$\scriptsize C_{12}H_{26} \: \underset{350^oC}{\overset{Al_2O_3}{\rightarrow}} \: X \: + \: C_{8}H_{18}$$

(i) What type of reaction is represented by the equation?

(ii) Write the molecular formula of X.

(iii) Draw the structure of two isomers of X.

(iv) Name the isomers drawn in (a)(iii).

1.          But – 1 – ene
2.           But – 2 – ene
3.           2 – methylpropene

(v) Write a balanced equation for the reaction between X and hydrogen.

Answer:  C4H8  +  H2  → C4H10

(b) Describe one test for fats.

Answer: place a small amount of the substance on a filter paper and melt/warm/rub. The formation of a translucent spot on the filter paper indicates the presence of fats

OR

Treat with sudan III reagent. Add sudan III to fat. A red colour is observed.

(c) Sulphur (IV) oxide is converted to tetraoxosulphate (VI) acid according to the following equation:

2SO2(g)   +  O2(g)   +  2H2O(l)   →   2H2SO4(aq)

If 1.5 moles of oxygen reacts with sulphur (IV) oxide, calculate the mass of tetraoxosulphate (VI) acid produced.

[H = 1.0; O = 16.0; S = 32.0].

Solution

Molar mass of H2SO4 = (1 x 2 + 32 + 16)

= 98gmol-1

From the equation

1 mole of O2 forms 2 moles of H2SO4

1.5moles of oxygen will produce 3 moles of H2SO4

Mass of 3 moles of H2SO4 = 3 x 98

= 294g

Or

1 mole O2 →  2 x 98g  H2SO4

1.5 mole O2   →  1.5 x 2 x 98

= 294g

(d) Consider the following neutralization reactions:

CH3 COOH  +  NaOH → CH3 COONa  + H2O;  ∆H1

CH3 COOH  + NH4 OH → CH3 COONH4  +  H2O; ∆H2

NaOH + HCI   →   NaCl  + H2O; ∆H3

(i) Arrange the enthalpy changes for the reactions in order of increasing magnitude.

Answer: ∆H2   <     ∆H1      <  ∆H3

(ii) Explain briefly your order in (d)(i).

Answer:  Naturalization involves the combination of H+ and OH- ions to give a specific amount of energy. The greater the ionization the greater the energy given off.

∆H2 involves a weak acid and a weak base

∆H1 involves a strong base and a weak acid

∆H3 involves a strong acid and a strong base

(e) Consider the following substances.

Cu(s) , BeCl2(s), NaH(s), HF(s) and CCl4(l)

State the substance(s) which;

(i) can conduct electricity

(ii) is/are soluble in water.

Answer:  NaH(g) , BeCl2(s), HF(l) are soluble in water.

## Question 4

(a)(i) Describe briefly the industrial preparation of ammonia.

Answer: Dry nitrogen (N2(g)) and dry hydrogen (H2(g))are mixed together in volume proportion if 1:3 respectively. The reaction takes place over a pressure of 200 - 500 atm with finely divided iron as a catalyst and at a temperature of 450°C — 500°C.

(ii) Write a balanced equation for the reaction in (a)(i).

Answer:  3H2(g)  +  N2(g)   $$\leftrightharpoons$$ 2NH3(g)

(iii) State one way of increasing the yield of ammonia in 4(a)(i).

• Continuous removal of ammonia as it is formed
• Liquefying the ammonia
• Low temperature
• High pressure

(iv) State two uses of ammonia.

• Manufacture of fertilizers
• As a precipitating agent
• Manufacture of trioxonitrale (v) acid
• Manufacture of trioxocarbonate (iv), textiles, pesticides
• Used in detergents/cleaning agent
• Manufacture of plastics
• Laundry works
• As a refrigerant

(b) Describe briefly, one chemical test for each of the following gases in the laboratory:

(i)Hydrogen

Answer:  Hydrogen – when a lighted wooden splint is brought to the tip of a test tube containing the gas, the gas gives up a ‘pop’ sound.

(ii) Carbon (IV) oxide

Answer:  Carbon (iv) oxide – Bubble gas through lime water {Ca(OH)2} solution. The lime water turns milky.

(iii) Oxygen

Answer:  Oxygen – when a glowing {wooden} splint is dipped into a test tube of the gas, the splint rekindles.

(c)(i) State the composition of water gas.

Carbon(ii) oxide {CO}

Hydrogen {H2

(ii) List two uses of water gas.

It is used as fuel

Industrial manufacture of hydrogen, method, butanol

(d) Describe briefly a simple experiment to determine the type of hardness in a sample of water.

• Boil a sample of water

If it forms lather easily, then it is temporary hardness.

If it forms lather with difficulty or does not form lather easily, then it is permanent hardness.

## Question 5

(a)(i) Describe briefly how trioxonitrate (V) ions could be tested for in the laboratory.

Answer: Add freshly prepared iron (ii) tetraoxosuplhate (vi) {FeSO4} solution followed by conc. H2SO4.  A brown ring is observed.

OR

Add concentrated H2SO4 to the substance, then heat. Brown fumes of NO2 is observed.

OR still

Mix substance with copper and con. H2SO4 and warm. A reddish-brown fume of NO2 is observed.

(ii) State two uses of each of the following compounds:

I. Sodium chloride;

• Food seasoning
• Food preservative
• Soap production
• Crude oil refining
• Water softener
• Production of Chlorine/hydrogen/sodium metal
• Used in tannery industry

II. Sodium trioxocarbonate (IV).

• Manufacture of glass/borax
• In detergents/soap
• Manufacture of paper
• Softening hard water
• Manufacture of NaOH

(b) Write balanced equations for the reactions involved in the extraction of iron in the blast furnace.

C(s)  +  O2(g) →  CO2(g)

CO2(g)  + C(s) → 2CO(g)

Fe2O3  +  3CO(g) → 2Fe  +  3CO2(g)

Fe3O4  +  4CO → 3Fe  +  4CO2

CaO   +  SiO2 → CaSiO3

CaCO3 →  CaO  +  CO2

(ii) State Faraday's first law of electrolysis.

Answer: It states that the mass of an element discharged at the electrode is directly proportional to the quantity of electricity passed.

(iii) State two applications of electrolysis.

• Electroplating of metals
• Extraction of metals
• Purification of metals
• Preparation of compounds/elements

(c) Concentrated tetraoxosulphate (VI) acid is added to sugar crystals in a beaker. State what would be observed. Explain briefly your answer.

• Beaker becomes warm/formation of black solid
• The concentrated acid dehydrates the sugar
• A mass of carbon/black soot/sugar charcoal is formed.
• Reaction is exothermic

(d) Write an equation for the reaction of zinc powder with:

(i) Dilute tetraoxosulphate (VI) acid;

Answer: Zn(s) + H2SO4(aq) → ZnSO4(aq)  +  H2(g)

(ii) Concentrated tetraoxosulphate (VI) acid.

Answer: Zn(s)  +  2H2SO4(l) → ZnSO4(aq)  +  SO2(g)  +  2H2O(l)

(e) What property of concentrated tetraoxosulphate (VI) acid is shown in (d)(ii)