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.

Answer: 

 

(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.

Answer:

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

Answer:

Examples of primary cells

Examples of secondary cells

 

(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.

Answer:

 

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

Answer:

 

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

Answer:

(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.

Answer:

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

Answer: 

 

(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 \(\scriptsize  ^{37}_{17}X\)

Answer: Isotopy

(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

Answer:

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?

Answer: Catalytic cracking

(ii) Write the molecular formula of X.

Answer: X is C4H8

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

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

Answer: 

  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

Answer:  Cu(s) 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).

Answer: 

(iv) State two uses of ammonia.

Answer: 

 

(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.

Answer: 

Carbon(ii) oxide {CO}

Hydrogen {H2

(ii) List two uses of water gas.

Answer: 

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.

Answer: 

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;

Answer:  

II. Sodium trioxocarbonate (IV).

Answer:  

 

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

Answer:  

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.

Answer:

 

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

Answer: 

 

(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)

Answer: Oxidizing   

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.

Answer: 

 

(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.

Answer:

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

Answer:

Examples of primary cells

Examples of secondary cells

 

(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.

Answer:

 

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

Answer:

 

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

Answer:

(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.

Answer:

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

Answer: 

 

(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 \(\scriptsize  ^{37}_{17}X\)

Answer: Isotopy

(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

Answer:

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?

Answer: Catalytic cracking

(ii) Write the molecular formula of X.

Answer: X is C4H8

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

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

Answer: 

  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

Answer:  Cu(s) 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).

Answer: 

(iv) State two uses of ammonia.

Answer: 

 

(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.

Answer: 

Carbon(ii) oxide {CO}

Hydrogen {H2

(ii) List two uses of water gas.

Answer: 

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.

Answer: 

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;

Answer:  

II. Sodium trioxocarbonate (IV).

Answer:  

 

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

Answer:  

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.

Answer:

 

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

Answer: 

 

(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)

Answer: Oxidizing