Chemsecret is an international nonprofit scientific platform

Section A: Answer all questions

1. The table below contains some information about two elements

a) Complete the table below(1Marks)

Elements	Protons	Neutrons	Electrons
Sodium	11	12
Oxygen		10	8

b) What is the isotopic mass (mass number) of oxygen? (1Marks)

c) Write the electronic configuration of sodium (1Marks)

d) Sodium reacts with oxygen to form sodium oxide

Write the balanced equation for the reaction (2Marks)

Answer

a) Before filling the empty space in the below table, you have to remember the relationship between, proton, neutrons and electron, which state that the number of electrons equal to number of protons for a given element.

Elements	Protons	Neutrons	Electrons
Sodium	11	12	11
Oxygen	8	10	8

b) Formula: Isotopic mass (mass number) = protons + neutrons

The Isotopic mass of Sodium= 8 +10=18

c) In order to get the electronic configuration of sodium, you have to know the atomic number of sodium which is equal to 11.

1^st method:

2^nd method: 2,8,1

3^rd method: 1s²2s²2p⁶3s¹

d) The balanced equation: 4 Na + O₂ → 2 Na₂O

2. Limestone (CaCO₃) is a mineral that is heated to produce lime(CaO). Lime is used in the manufacture of iron, Cement and Glass

a) What is the chemical name for limestone? (1Marks)

b) The decomposition of limestone is shown in the equation

CaCO₃⇌ CaO + CO₂

Calculate the mass of lime (CaO) that would be produced from 2kg of limestone(CaCO₃) (2Marks)

Relative atomic mass (C=12, O=16, Ca=40)

Answer

a) Chemical name of limestone (CaCO₃) is Calcium carbonate

b) CaCO₃ → CaO + CO₂

Calculate first, molecular mass of CaCO₃ = 40+12+(16×3) = 100g/mol

and molecular mass of CaO= 40 +16= 56g/mol

From the above equation

1moles of CaCO₃ produce 1mole of CaO

It means that, 100g of CaCO₃ produce 56g of CaO

And 1g of CaCO₃ will produce $\frac{56}{100}$ \frac{56}{100}of CaO

Then , 2000g of CaCO₃will produce $\frac{56}{100}$ \frac{56}{100}× 2000 = 1120 g of CaO

Therefore, the mass of Lime that is produced from 2kg of limestone is 1120g

3. Air is mixture of many gases some of which are shown in the table below

a) Complete the table below (1^1/2Marks)

Name	Chemical formula	Approximate percentage in air
Nitrogen
Carbon dioxide	CO₂
Inert gases	Ne. Ar etc.	1

b) Mention one way in which carbon dioxide is removed from the atmosphere and one way in which it is supplied to the atmosphere (2Marks)

Answer

a)

Name	Chemical formula	Approximate percentage in Air
Nitrogen	N₂	78
Carbone dioxide	CO₂	0.03
Inert gases	Ne, Ar, etc	1

b) One way CO₂ is removed from the atmosphere is:

· Forestation

· Improve waste management system

· Use biomass for the production of fuels and product

One way in which CO₂ is supplied to the atmosphere is:

· Waste

· Combustion of fossil fuel

· Respiration

4. Hydrogen peroxide (H₂O₂) is used as a bleaching agent and as a source of oxygen

a) What is a bleaching agent? (1Marks)

b) Write a balanced equation for the production of oxygen from hydrogen peroxide (1Marks)

c) The rate of decomposition of H₂0₂ is very slow at room temperature, suggest two ways of speeding up the decomposition (2Marks

Answer

a) A bleaching agent is a chemical substance that can whiten or decolorize other substances

b) 2H₂O₂ →^MnO2 2H₂O + O₂

c) Two ways of speeding up the decomposition are:

· It can be sped up by increasing the temperature

· Use the catalyst for example MnO₂, silver and enzymes

· It can be sped up also by increasing the concentration of hydrogen peroxide

5. The following is an exothermic reaction and takes place in the presence of iron as a catalyst

N_2(g)+ 3 H_2(g) ⇌ 2NH_3(g)

a) What do you understand by Exothermic reaction? (1Marks)

b) What does the sign ⇌ mean in the equation? (1Marks)

c) What would be the effect on the amount of ammonia produced if the pressure was increased beyond 600? (1Marks)

d) What would be the effect on the amount of ammonia produced if the temperature was increased beyond 600^oC (1Marks)

Answer

a) Exothermic reaction is the reaction that releases heat, causing the temperature of the immediate surroundings to rise.

b) ⇌ means reversible reaction

c) The increase of pressure lead to the increase the amount of ammonia produced

d) When the temperature increase, the amount of ammonia produced will decrease.

6. Some reaction involving carbon dioxide CO₂ are shown below study the chart and answer the equation that follow

a) What are chemical names and chemical formulae for substance A, B and C? (3Marks)

b) Write a balanced chemical equation for the formation of substance C from CO₂(1Marks)

Answer

Symbol	Name	Chemical formula
A	Carbonic acid	H₂CO₃
B	Calcium Carbonate	CaCO₃
C	Sodium hydrogen Carbonate	NaHCO₃

b) The balanced equation: 2 NaOH_(aq) + 2CO₂ → 2NaHCO₃

7. The molecular formulae of two organic substances x and Y are given below

X= C₄H₁₀, Y= C₄H₈

a) Write two structural formulae of substance X and their chemical names (2Marks)

b) Write one structural formulae of Y and given its chemical name (1Marks)

c) Write a balanced chemical equation for the reaction of X with Chlorine. (1Marks)

Answer

a) Structural formula of X:

1. Butane

2. 2-methyl propane

b) Structural formula of Y: But-2-ene

8. The diagram below shows the direction of movement of ions during electrolysis of dilute sodium chloride solution (NaCl)

a) Which of the electrodes A and B is the Anode? (1Marks)

b) Which element (substance) is formed at the electrode B?

Write an equation for the reaction taking place at B (2Marks)

c) At electrode A, there are two different gases that are likely to form. What chemical test would you use to show the presence of gas which is the main product? (1Marks)

Answer

a) The electrode A is an Anode

b) Hydrogen gas will be formed at electrode B

Equation: 2H⁺+ 2e^-→ H₂

c) The main product is oxygen, it is tested using a glowing splint, it relights a glowing splint.

9. The table below shows the reactivity of some metals with water and dilute sulphuric acid. Study the table very carefully and answer the questions that follow.

Metal	Reaction with water	Reaction with dilute sulphuric Acid
Calcium	Hydrogen formed fast	Not advisable
Copper	No reaction	No reaction
Iron	Rust was formed slowly	Hydrogen formed slowly
Magnesium	Hydrogen formed slowly	Hydrogen formed quickly

a) Which of the above substances is the most reaction? (1Marks)

b) Which of the above substances is the least reactive? (1Marks)

c) Why is it not advisable to react calcium with dilute sulphuric acid? (1Marks)

d) How would you show that hydrogen is evolved when magnesium reacts with dilute sulphuric Acid? (1Marks)

Answer

a) The reaction of Calcium with water and the reaction of Magnesium with diluted sulfuric acid

b) The reaction of Copper with water

c) The reaction of sulfuric acid with calcium metal produces a coating of calcium sulfate (CaSO4) on the metal. Since the calcium sulfate is insoluble in water, the coating acts as a protective layer thus preventing further attack on the metal by the acid means that the reaction will be stopped.

d) It burns with a pop sound

10. Copper can be extracted from an ore called copper pyrites whose formula is CuFeS₂

a) Give the chemical names of the elements present in copper pyrites (1^1/2Marks)

b) Copper is obtained by heating the ore in controlled supply of air with sand (SiO₂) the reaction is

2 CuFeS₂+ 5O₂ +2 SiO₂→2 Cu + 4 SO₂ +2 FeSiO₃

How much copper would be obtained by heating 36.7 g of CuFeS₂? Relative atomic mass Cu=63.5, Fe=56, S=32. (3Marks)

Answer

a) Chemical names present in copper pyrites are:

· Copper

· Iron

· Sulphur

b) The relative molecular mass of CuFeS₂= 63.5 +56 +(32×2) =183.5g/mol

from this reaction 2 CuFeS₂+ 5O₂ +2 SiO₂ → 2 Cu + 4 SO₂ +2 FeSiO₃

2moles of copper pyrites gives 2 moles of copper

Thus, (2 x 183.3) g of CuFeS₂ gives (2 x 63.5) g of Cu

and 1g of CuFeS₂ will produce $\frac{(2 \times 63.5)}{(2 \times 183.5)}$ \frac {(2×63.5) }{(2×183.5)} g of Cu

then 36.7 g of CuFeS₂ will produce $\frac{(2 \times 63.5)}{(2 \times 183.5)}$ x 36.7 = 12.7 g

As conclusion 36.7 g of CuFeS₂ will produce 12.7g of Cu

11. The diagram below shows some changes of states. The direction of the arrow shows what change of state it is.

a) Names changes or process 1,2,3 and 4. (2Marks)

b) States two differences between a solid and gas (2Marks)

Answer

1. Condensation

2. Evaporation

3. Melting

4. Sublimation

b) Two differences between a solid and a gas

· Gas has no definite shape (takes the shape of its container) while solid has definite shape (rigid)

· Gas has no definite volume while solid has definite volume

· The particles move in random motion with little or no attraction to each other in gas state while in solid particles vibrate around fixed axes

12. Calculate the percentage composition of oxygen in one mole of aluminium sulphate Al₂(SO4)₃. Relative atomic masses are: Al=27, S=32, O=16. (3Marks)

Answer

The relative molecular mass of Al₂(SO₄)₃ : (27 × 2) + (32×3) + (16 ×12) = 342g/mol

The relative molecular mass of oxygen in Al₂(SO₄)₃: (16 ×12) g/mol = 192g/mol

The percentage (%) of oxygen: $\frac{(192 \times 100)}{342}$ \frac{(192×100)}{342} = 56.1%

13. Study the table below and answer the questions that follow

Substance	Melting point ^oC	Boiling point ^oC
Methanol	-94	65
Ethanol	-117	79
Water	0	100

a) Which of the three substances is the most volatile? (1Marks)

b) Which substance that becomes a liquid at the highest temperature? (1Marks)

c) Write the chemical formula for ethanol (1Marks)

Answer

a) The most volatile substance is Methanol

b) Water is the one that will become liquid at high temperature

c) The chemical formula of Ethanol:C₂H₅OH or CH₃-CH₂-OH

14. An organic compound contains 52.17% of carbon, 13% of hydrogen the rest being oxygen.

a) Calculate the empirical formula of the compound. (3Marks)

b) Given that the relative molecular mass is 92, determine its molecular formula. (2Marks)

$\frac{92}{46}$

Answer

Since you know the percentage composition, it is convenient to assume that you have 100 g of the compound. This means that you have 52.17 g of carbon and 13g of hydrogen and the mass percentage of oxygen will be 100g-( 52.17+13) g = 34.83g. Convert each mass to moles. The number of moles can then be converted into a lowest terms ratio of the elements to get the empirical formula.

Elements	Mass Percentages	Molar mass	Number of moles	Simplest ratios( molar amount divided by lowest molar amount )
C	52.17	12	$\frac{52.17}{12} = 4.3475$ { \frac {52.17} {12}}=4.3475	$\frac{4.3475}{2.1769} = 2$ { \frac {4.3475} {2.1769}}=2
H	13	1	$\frac{13}{1} = 13$ { \frac {13} {1}}=13	$\frac{13}{2.1769} = 6$ { \frac {13} {2.1769}}=6
O	34.83	16	$\frac{34.83}{16} = 2.1769$ { \frac {34.83} {16}}=2.1769	$\frac{2.1769}{2.1769} = 1$ { \frac {2.1769} {2.1769}}=1

Therefore, the empirical formula is C₂H₆O

The molecular formula = (Empirical formula)_n

n= Molecular Mass / Molecular weight

The empirical weight = (12×2) +(1×6) +16

n = 92:46=2

Thus, Molecular formula = (C₂H₆O) 2= C₄H₁₂O₂

SECTION B: Answer three questions in this section.

15. The ore haematite contains iron oxide. The following flow diagram (chart) shows how iron is extracted from its ore haematite. Study the diagram and answer the question that follow

a) Name substance X and Y (2Marks)

b) The chemical reaction for the formation of iron is

Fe₂O_3(S) + CO_(g) → 2Fe_(s) + 3CO_2(g)

From the above equation, identify which substance is a reducing agent and which substance is an oxidizing agent. (2Marks)

c) Give the chemical names of three substances in the slag (3Marks)

d) The blast furnace should be located (built) where conditions are suitable for an industry. Mention three conditions to be consider before deciding where to put the industry. (3Marks)

Answer

a) X → Coke (Carbon), Y→ limestone

b) Fe₂O₃+ 3CO → 2Fe + 3 CO₂

· Fe₂O₃ is an Oxidizing agent

· CO is a Reducing agent

c) Slag is Calcium silicate, CaSiO₃

Chemical names substances in the slag are:

· Calcium

· Silicon

· Oxygen

d) The following parameter must be considered while you are choosing the place to build industry: (choose three of them)

· Availability of Raw materials

· Nearness to operating power

· Availability of Water

· Climate

· Transportation

· Governmental policy

· Nearness to the Market

16. Below is a simplified flow diagram for the manufacture of sulfuric Acid. Study the diagram and answer the question that follow.

a) Write a balanced equation for the formation of Sulphur trioxide from oxygen and sulfur dioxide (2Marks)

b) Starting with 50cm³ of Sulphur dioxide and 50cm³ of oxygen

i. Calculate the volume of Sulphur trioxide formed (1Marks)

ii. Calculate the volume of oxygen that remains unreacted (1Marks)

c) Name the catalyst used in the formation of sulfur trioxide (1Marks)

d) Write an equation to show how concentrated sulphuric acid reacts with (2Marks)

i. Copper

ii. Carbon

e) Name the two environmental problems that can be caused by the presence of Sulphur dioxide in the atmosphere. (1Marks)

Answer

a) The balanced equation: 2 SO₂ + O₂ → 2 SO₃

b) From the equation in (a):

2Volume of SO₂ + 1Volume of O₂ produce 2Volume of SO₃

1Volume of SO₂+ Volume of O₂ will produce 1Volume of SO₃

From the above statement, 50 Cm³ of SO₃+ × 50 Cm³of O₂ will produce 50Cm³ of SO₃

50Cm³ of SO₂ + 25 Cm³of O₂ → 50 Cm³ of SO₃

1. the Volume of SO₃ that will be produced is 50 Cm³

2. the Volume of O₂ that remained unreacted is 50-25=25 Cm³

c) Catalyst used in formation of sulfur trioxide is, V₂O₅

1. Cu + 2H₂SO₄ → CuSO₄+ SO₂ + 2H₂O

2. C + 2H₂SO₄→ 2SO₂ + CO₂ + 2H₂O

e) Acid rain:

· It causes the deforestation

· acidify waterways to the detriment of aquatic life

· corrode building materials and paints

· Dissolve minerals away (soil infertility)

17. The following is a periodic table showing some elements. Use the table and the elements shown to answer the equations.

a) How many electrons does an atom of element F contain? (1Marks)

b) Write the electronic configuration of the element Ca. (1Marks)

c) Give the symbols of the two elements that belong to alkaline metals (2Marks)

d) Give the formula of the compound formed between elements Mg and P. (1Marks)

e) Select one element that will form a basic oxide, one element that will form an acid oxide and one element that will form an amphoteric oxide (3Marks)

f) Use dot and cross diagrams to show the boding in CH₄(2Marks)

Answer

a) Element F has 9 electrons

b) Electronic configuration of Ca is:

1^st method: 2,8,8,2

2^nd method: 1s²2s²2p⁶3s²3p⁶4s²

c) Mg, Ca

d) Mg₃P₂

e) Mg, S, Al

f) Dot and cross diagram bonding in methane

18. Starting with zinc powder and copper II sulphate solution,

a) Explain how you would obtain a pure sample of zinc sulphate and a pure sample of copper powder. (8Marks)

b) Write an ionic equation from the following chemical reaction. (2Marks)

Answer

a) Zinc powder and copper II sulphate solution are mixed. The reaction takes place

i.e. Zn_(s)+ CuSO_{4 (aq)}→ ZnSO_4(aq) + Cu_(s)

after the reaction is over, filtration is done to separate zinc sulphate solution from a brown solid of copper.

b) ZnSO_4
(aq)+ BaCl_{2 (aq)}→ BaSO_4(s)+ ZnCl_{2 (aq)}

Ionically, Zn²⁺_(aq)+ SO^2-_{( aq)} + Ba²⁺ _(aq)+ 2Cl^-_(aq) → BaSO_4(s)+ Zn²⁺_(aq)+ 2 Cl^-_(aq)

Ionic equation: Ba²⁺_{( aq)}+ SO₄²⁺_(aq)→ BaSO_4(s)

19. a) With the aid of well labelled diagram, describe an experiment to prepare hydrogen chloride gas (HCl) from sodium chloride (NaCl) (8Marks)

b) Write equation to show how HCL reacts with

i. Ammonia gas (1Marks)

ii. Manganese IV oxide (MnO₂) (1Marks)

Answer

a) Schematically preparation of hydrogen chloride

b) i. NH₃ + HCl → NH₄Cl

ii. MnO₂ + 4HCl → MnCl₂ + Cl₂+ 2H₂O

Section C: Answer only one question from this section.

20. To determine the concentration of solution of sulphuric acid, 25 cm³ of sulphuric acid was titrated with 2M potassium hydroxide solution. The volume of potassium hydroxide required was 34cm³

a) State three pieces of apparatus that would be used in this experiment. (3Marks)

b) How would you tell that the acid is completely neutralized by the base?

Given that the equation of the reaction is (1Marks)

c) Calculate the number of moles of potassium hydroxide used in the titration. (2Marks)

d) Calculate the concentration of sulphuric acid in g/dm³

Relative atomic mass is H=1, S=32, O=16, K=39 (5Marks)

e) Suggest two uses of sulphuric acid and two uses of potassium hydroxide. (4Marks)

Answer

a) Burret, pipette, Conical flask,

b) When the color of the indicator in the solution just changes.

c) 2KOH (aq) + H₂SO₄ (aq) → K₂SO₄ (aq) + 2H₂O(l)

Number of moles of KOH = $\frac{2 \times 34}{1000}$ \frac{2×34}{1000} = $\frac{68}{1000}$ \frac{68}{1000} = 0.068

d) Let n_a= 1; n_b=2; M_a=? M_b= 2; V_a= 25cm³; V_b=34Cm³

$\frac{n a}{n b} = \frac{M a \times V a}{M b \times V b}$ → $\frac{1}{2} = \frac{M a \times 25}{2 \times 34}$ { \frac{1} {2}=\frac {Ma×25} {2×34}} → 2Ma × 50 = 2 × 34

Ma= $\frac{2 \times 34}{2 \times 25}$ {\frac {2×34} {2×25}} = 1.36M

But concentration in g/dm³ = molarity × relative molecular mass

Relative molecular mass = (1×2) ÷ 32 + 16×4 = 98g/mol

Concentration in g/dm3 = 1.36 × 98 = 133.28g/dm³

e) Two uses of H₂SO₄

· It is used in petroleum refining to wash impurities out of gasoline

· It is used in production of fertilizers

· It serves as the electrolyte in the lead-acid storage battery commonly used in motor vehicles

· Preparation of salts

· It is used in manufacturing of chemicals.

· It used in making of synthetic detergent, dyes and pigment

· It is used in metal processing for example iron cleaning before plating it with zinc or tin

21. In an experiment to determine how hydrogen is produced when magnesium powder reacts with dilute hydrochloric acid, the volume of hydrogen produced was measured at different interval of time. The following results were obtained.

Time ( seconds)	0	5	10	20	30	40	50	60
Volume of H₂( cm³)	0	32	52	78	93	95	95	95

a) Plot a graph of volume of H₂ produced (on –y-axis) versus time (x-axis). (9Marks)

b) Why is the volume of H₂constant in the last three results? (1Marks)

c) Suggest a suitable instruments or piece of apparatus that can be used to measure the volume of H₂(1Marks)

d) Why is the volume of H₂ =0 Cm³ when time=0 seconds? (1Marks)

e) How would you prove that hydrogen gas is evolved in each experiment? (1Marks)

f) State two industrial uses of hydrogen gas. (2Marks)

Answer

b) The Volume of H₂ is constant because the reaction is over

c) Syringe

d) At 0 seconds, the reaction was not yet started

e) Using a burning splint, if a pop sound is observed, then the gas is hydrogen.

f) Hydrogen(H₂) is used in

· Production of fertilizers

· Foods processing

· Refining petroleum

· It also used as fuel

22. With the help of equations where possible, state the chemical test that would be used to distinguish each pair of the following substances and state the observation in each case. (3Marks each)

a) Zn(NO₃)_2
(aq) and Fe(NO₃)_3(aq)

b) NaCl_(aq) and Na₂CO_{3 (aq)}

c) Ethene and Ethanol

d) SO₂ and Cl₂(gases)

e) Pb(NO₃)_2(S) and Cu(NO₃)_2(S)

Answer

To distinguish between each of the following pairs of substances:

a) Zn(NO₃)_2
(aq) and Fe(NO₃)_3(aq)

Reagent: Ammonia solution (NH₄OH)

· With Zn^2+, a White precipitate which dissolves in excess ammonia solution will be observed. Zn²⁺ + 2OH^-→ Zn(OH)₂

· With Fe³⁺, a Brown precipitate insoluble in excess NH₃ (aq)(NH₄OH);

Fe³⁺ + OH^-→ Fe(OH)₃

b) NaCl_(aq) and Na₂CO_{3 (aq)}

Reagent: Acidified silver nitrate solution,

· With Cl-, a white precipitate observed, Ag+ + Cl- → AgCl (s)

· With CO₃^2- Effervescence, no precipitate formed CO₃^2- + 2H^- → CO₂ + H₂O

c) Ethene and Ethanol

Reagent: bromine water

Observations:

· With ethane, it decolorizes bromide water

· With ethanol, no observable change

d) SO₂ and Cl₂(gases)

Reagents: acidified KMnO₄ or K₂Cr₂O₇

Using KMnO4

· With SO₂, the purple color of KMnO₄ turns to Colorless

· With Cl₂, No Observable change

Using K₂Cr₂O₇

· With SO₂, the Color of K₂Cr₂O₇ Changes from orange to green

· With Cl₂, no observable change

e) Pb(NO₃)₂(S) and Cu(NO₃)₂(S)

Reagents: NaOH Solution or NH₃(aq)

Using NaOH Solution:

Observation:

· With Pb²⁺, a white precipitate soluble in excess NaOH solution to form a colorless solution. Pb²⁺+ 2OH^- → Pb(OH)₂ White precipitate

· With Cu²⁺, a blue precipitate in excess NaOH, Cu²⁺ + 2 OH- → Cu(OH)₂ blue precipitate

Using NH₃ (aq)

· With Pb^2+, A White precipitate insoluble in excess NH₃

· With Cu²⁺, Blue precipitate soluble in excess forming a deep blue solution.

Answer

US FDA Warning of Dangerous HCG Diet Products

Rwanda Food and Drugs Authority Issues Warning About Methanol Based Hand Sanitizer

Rwanda FDA advises the Buyers about Processed Products

USFDA Issues Warning Letter About ENDS Products