JAMB UTME - Chemistry - 2002
Vraag 1 Verslag
Which of the following gives a precipitate when treated with NaOH solution?
Antwoorddetails
When a substance reacts with NaOH solution to produce a solid or insoluble product, it is said to give a precipitate. We can use this knowledge to determine which of the given options will give a precipitate when treated with NaOH solution. Option A, AlCl3, will give a precipitate when treated with NaOH solution. This is because AlCl3 is an acidic salt and will react with the basic NaOH to produce a gelatinous precipitate of aluminium hydroxide (Al(OH)3), which is insoluble in water. Option B, NH4Cl, will not give a precipitate when treated with NaOH solution because NH4Cl is a salt of a strong acid (HCl) and a weak base (NH3), and it will not react with NaOH to form a precipitate. Option C, CH3COONa, will not give a precipitate when treated with NaOH solution because it is a salt of a weak acid (acetic acid) and a strong base (NaOH). Therefore, it will not react with NaOH to form a precipitate. Option D, Na2CO3, will give a precipitate when treated with NaOH solution. This is because Na2CO3 is a basic salt and will react with the acidic NaOH to produce a white precipitate of sodium carbonate (Na2CO3), which is insoluble in water. Therefore, the option that gives a precipitate when treated with NaOH solution is option A, AlCl3, and option D, Na2CO3. The answer is not uniquely defined.
Vraag 2 Verslag
The substance least considered as a source of environmental pollution is
Antwoorddetails
All of the listed substances can potentially cause environmental pollution, but the substance least considered as a source of environmental pollution is silicate minerals. Silicate minerals are naturally occurring minerals that make up a large portion of the Earth's crust and are typically not harmful to the environment. On the other hand, lead compounds, organophosphorus compounds, and uranium are all known to be harmful to the environment and can cause pollution. Lead can contaminate soil and water, causing damage to wildlife and humans who come into contact with it. Organophosphorus compounds are often used as pesticides and can have negative effects on the health of humans and animals, as well as on the environment. Uranium is a radioactive element that can cause contamination of soil, water, and air, leading to harmful effects on living organisms. In summary, while all of the listed substances can potentially cause environmental pollution, silicate minerals are the least considered as a source of environmental pollution due to their natural occurrence and low likelihood of causing harm.
Vraag 3 Verslag
The atom of an element X is represented as \( {}^{y}_{z}X \). The basic chemical properties of X depend on the value of
Antwoorddetails
The symbol used to represent an atom of an element X is:
Here, 'X' represents the chemical symbol of the element and 'z' and 'y' are the atomic number and mass number of the element respectively.
The basic chemical properties of an element depend on the number of protons (i.e., atomic number) in its atoms. This is because the number of protons determines the number and arrangement of electrons in an atom, and the chemical properties of an element are primarily determined by the behavior of its electrons.
Therefore, in this case, the basic chemical properties of element X depend on the value of 'z', which represents the atomic number of X.
Vraag 4 Verslag
In the reaction E+F ↔ G+H, the backward reaction is favoured if the concentration of
Antwoorddetails
Vraag 5 Verslag
25cm3 of a 0.2 mol dm3 solution of Na2CO3 requires 20 cm3 of a solution of HCl for neutralization. The concentration of the HCl solution is
Antwoorddetails
The question describes a neutralization reaction between a solution of Na2CO3 and HCl. The volume of HCl required for neutralization is given as 20 cm3. From the balanced equation for the reaction, we know that 1 mole of Na2CO3 reacts with 2 moles of HCl. Therefore, the moles of HCl required for neutralization can be calculated as follows: moles of HCl = (moles of Na2CO3) x 2 moles of Na2CO3 = concentration x volume / 1000 Substituting the given values, we get: moles of Na2CO3 = 0.2 x 25 / 1000 = 0.005 moles of HCl = 0.005 x 2 = 0.01 The concentration of HCl solution can then be calculated by dividing the moles of HCl by the volume of HCl solution used: concentration of HCl = moles of HCl / volume of HCl solution concentration of HCl = 0.01 / 20 / 1000 concentration of HCl = 0.5 mol dm^-3 Therefore, the concentration of the HCl solution is 0.5 mol dm^-3.
Vraag 6 Verslag
The most important use of hydrogen is in the
Antwoorddetails
The most important use of hydrogen is in the manufacture of ammonia. Ammonia is a vital chemical used in the production of fertilizers, which are essential for modern agriculture. Hydrogen gas is combined with nitrogen gas to produce ammonia in a process called the Haber process. This process is highly dependent on the availability of hydrogen gas, making it the most important use of hydrogen. While hydrogenation of oils, manufacture of methyl alcohol, and ethyl alcohol are also important uses of hydrogen, they are not as critical as the production of ammonia for the global economy.
Vraag 7 Verslag
Which of the following decreases when a given mass of gas is compressed to half its initial volume?
Antwoorddetails
When a given mass of gas is compressed to half its initial volume, the average intermolecular distance decreases. This is because when the volume of the gas is decreased, the molecules are brought closer together, which leads to a decrease in the average distance between them. The frequency of collisions between the gas particles will increase as they are closer together, and the number of molecules present in the gas will remain the same, as the mass of the gas has not changed. The atomic radius of each particle also remains the same, as compression does not affect the size of individual gas particles.
Vraag 8 Verslag
A hydrogen atom which has lost an electron contains
Antwoorddetails
A hydrogen atom which has lost an electron contains only one proton. When an atom loses an electron, it becomes a positively charged ion. In the case of a hydrogen atom, which normally has one electron and one proton, the loss of an electron leaves only the proton in the nucleus. Protons are positively charged particles found in the nucleus of an atom, while electrons are negatively charged particles that orbit the nucleus. Since the hydrogen atom has lost an electron, the number of negatively charged particles no longer matches the number of positively charged particles, resulting in an ion with a positive charge. The other options listed, such as one neutron only, one proton and one neutron, or one proton, one electron, and one neutron, do not describe the structure of a hydrogen ion. A hydrogen ion contains only one positively charged proton and no electrons.
Vraag 9 Verslag
The processes which return carbon (IV) oxide to the atmosphere include
Antwoorddetails
The processes which return carbon (IV) oxide to the atmosphere include respiration, decay, and combustion. Respiration is the process by which living organisms, including plants and animals, release carbon dioxide into the atmosphere as they break down food molecules to release energy. Decay is the breakdown of organic matter by microorganisms, which also releases carbon dioxide. Combustion, on the other hand, is the burning of fuels, such as coal and oil, which releases carbon dioxide into the atmosphere as a byproduct of the reaction. While other processes, such as photosynthesis, transpiration, and ozone depletion, may play important roles in the carbon cycle, they do not directly return carbon dioxide to the atmosphere in the same way that respiration, decay, and combustion do.
Vraag 11 Verslag
When dissolved in water, NaOH flakes show
Antwoorddetails
When dissolved in water, NaOH flakes show an exothermic change. This means that heat is released during the dissolution process. The reason for this is that the interaction between NaOH and water molecules releases energy in the form of heat. This can be easily observed by touching the container in which the dissolution is taking place. It will feel warm to the touch due to the energy being released. Therefore, the correct option is "An exothermic change."
Vraag 12 Verslag
The property which makes alcohol soluble in water is the
Antwoorddetails
The property that makes alcohol soluble in water is hydrogen bonding. Hydrogen bonding is a special type of intermolecular force that occurs between a hydrogen atom bonded to an electronegative atom, such as oxygen or nitrogen, and another electronegative atom in a nearby molecule. Water molecules contain hydrogen bonding sites on their oxygen atoms, which can form hydrogen bonds with the hydroxyl (-OH) group of alcohols. This hydrogen bonding attraction allows alcohols to dissolve in water and form a homogeneous mixture.
Vraag 13 Verslag
Which of these pairs are synthetic and natural macromolecules respectively?
Antwoorddetails
Vraag 16 Verslag
The reaction of an alkene with hydrogen in the presence of catalyst is
Antwoorddetails
The reaction of an alkene with hydrogen in the presence of a catalyst is an addition reaction. This is because the double bond of the alkene is broken, and the hydrogen atoms are added to the molecule in a process called hydrogenation. No atoms are lost or gained, and the reaction is typically exothermic, releasing energy.
Vraag 17 Verslag
CU2S(s) + O2(g) → 2CU(s) + SO2(g)
What is the change in the oxidation number of copper in the reaction above?
Antwoorddetails
Vraag 18 Verslag
A compound contains 31.91% potassium, 28.93% chlorine and the rest oxygen. What is the chemical formula of the compound?
[K = 39, Cl = 35.5, O = 16]
Antwoorddetails
To find the chemical formula of the compound, we first need to determine the ratio of the elements in it. From the given percentages, we can assume that we have 100 grams of the compound, and then convert the percentages to grams: - Potassium (K) = 31.91 grams - Chlorine (Cl) = 28.93 grams - Oxygen (O) = 100 - 31.91 - 28.93 = 39.16 grams Next, we need to determine the ratio of the elements in terms of their moles. To do this, we need to divide each element's mass by its molar mass: - K: 31.91 g / 39.1 g/mol = 0.815 mol - Cl: 28.93 g / 35.5 g/mol = 0.816 mol - O: 39.16 g / 16 g/mol = 2.45 mol We can see that the ratio of K to Cl to O is approximately 1:1:3. Therefore, the chemical formula of the compound is KClO3, option (D). Each formula unit of KClO3 contains one potassium atom (K), one chlorine atom (Cl), and three oxygen atoms (O).
Vraag 19 Verslag
Hydrogen is readily released when dilute hydrochloric acid reacts with
Antwoorddetails
Hydrogen is readily released when dilute hydrochloric acid reacts with sodium (Na). When an acid reacts with a metal, it typically produces hydrogen gas and a salt. In the case of dilute hydrochloric acid and sodium, the reaction produces sodium chloride (NaCl) and hydrogen gas (H2). The reaction between dilute hydrochloric acid and sodium can be represented by the following equation: 2HCl(aq) + 2Na(s) → 2NaCl(aq) + H2(g) In this reaction, the hydrogen ions (H+) from the acid react with the metal atoms from the sodium (Na) to form hydrogen gas (H2). The sodium ions (Na+) combine with the chloride ions (Cl-) from the acid to form sodium chloride (NaCl), which dissolves in the water. The other options listed, such as Cu, Au, and Ag, do not react with hydrochloric acid to produce hydrogen gas. These metals have different chemical properties and reactivities compared to sodium, and would react differently with hydrochloric acid.
Vraag 20 Verslag
Three drops of a 1.0 mol dm-3 solution of NaOH are added to 20 cm3 of a solution of pH 8.4. The pH of the resulting solution will be
Antwoorddetails
When NaOH is added to a solution, it reacts with water to form hydroxide ions (OH-) and sodium ions (Na+). The OH- ions can then react with hydrogen ions (H+) in the solution, causing the pH to increase. In this case, three drops of a 1.0 mol dm-3 solution of NaOH are added to a solution of pH 8.4. The amount of NaOH added is very small compared to the volume of the solution, so we can assume that the amount of water in the solution remains relatively constant. The reaction between NaOH and water can be written as follows: NaOH + H2O → Na+ + OH- + H2O The OH- ions produced by the reaction will react with the H+ ions in the solution: H+ + OH- → H2O This will cause the concentration of H+ ions in the solution to decrease, and the pH to increase. The exact change in pH will depend on the amount of H+ ions in the original solution and the amount of OH- ions produced by the reaction with NaOH. However, since the pH of the original solution is already quite high at 8.4, the addition of a small amount of NaOH will not cause a significant increase in pH. Therefore, the pH of the resulting solution will be greater than 8.4, but not by a significant amount. Therefore, the correct answer is "Greater than 8.4".
Vraag 21 Verslag
The Arrhenius equation expresses the relationship between the speed of a reaction and its
Antwoorddetails
Vraag 23 Verslag
Tetraoxosulphate (VI) acid burns the skin by
Antwoorddetails
Tetraoxosulphate (VI) acid, also known as sulfuric acid, is a strong acid that can burn the skin upon contact. The reason it burns the skin is due to its ability to react with and destroy the proteins and lipids that make up our skin. This happens through a process called dehydration, where sulfuric acid removes water molecules from the skin, causing it to dry out and become damaged. Furthermore, sulfuric acid is highly exothermic, which means it releases a large amount of heat when it reacts with other substances. This can also contribute to the burning sensation and damage to the skin. Therefore, the correct answer is "Dehydration".
Vraag 24 Verslag
Ethanol can easily be produced by
Antwoorddetails
Ethanol can easily be produced by fermentation of starch. Fermentation is a process that involves the breakdown of sugar by microorganisms such as yeast or bacteria in the absence of oxygen. In the case of ethanol production, the sugar source can be derived from various agricultural crops such as corn, sugarcane, or even from waste products like fruit peels or bread. During the fermentation process, enzymes produced by the microorganisms break down the complex sugar molecules into simpler compounds, including ethanol and carbon dioxide. The ethanol produced can be purified and concentrated through distillation to obtain the final product. This process is relatively simple and can be carried out on a large scale, making it a popular method for ethanol production. The other options listed, such as the distillation of starch solution, catalytic oxidation of methane, or destructive distillation of wood, are not as commonly used for ethanol production.
Vraag 25 Verslag
\( \mathrm{PCl_5(g) \rightarrow PCl_3(g) + Cl_2(g)} \)
In the reaction above, an increase in pressure will
Antwoorddetails
Vraag 26 Verslag
A fixed mass of gas has a volume of 92cm3 at 3 oC. When will be its volume at 18 oC if the pressure remains constant
Antwoorddetails
This is a problem about the ideal gas law, which relates the pressure, volume, temperature, and amount of a gas. The equation for the ideal gas law is PV = nRT, where P is the pressure, V is the volume, n is the number of moles of gas, R is the ideal gas constant, and T is the temperature in Kelvin. To solve this problem, we are given the initial volume V1, initial temperature T1, and final temperature T2. We are also told that the pressure remains constant, which means that P1 = P2. Since the amount of gas is fixed, we can assume that n1 = n2. Using the ideal gas law, we can write: P1V1 = n1RT1 P2V2 = n2RT2 Since n1 = n2, we can simplify to: P1V1/T1 = P2V2/T2 We can solve for V2 by rearranging the equation: V2 = V1(T2/T1) Plugging in the given values, we get: V2 = 92 cm^3 * (291 K / 276 K) ≈ 97.0 cm^3 Therefore, the answer is 97.0 cm^3.
Vraag 27 Verslag
Iron is often galvanized in order to
Antwoorddetails
Iron is often galvanized in order to protect it against corrosion. Galvanizing is a process of coating a layer of zinc on the surface of iron or steel to prevent the formation of rust. Zinc is more reactive than iron and when it is exposed to air and water, it forms a layer of zinc oxide which acts as a protective layer for the underlying iron. This prevents the iron from rusting and increases its lifespan. Galvanizing is commonly used in outdoor applications where the iron or steel is exposed to the elements and is susceptible to corrosion.
Vraag 28 Verslag
An example of element that can catenate is
Antwoorddetails
The term "catenate" means to link together, and in chemistry, it refers to the ability of an element to form long chains or rings by bonding with other atoms of the same element. An element that can catenate is one that has a strong tendency to form such chains or rings. Out of the options given, the element that can catenate is carbon. Carbon is known for its unique ability to form long chains of carbon atoms, which can form the backbone of complex molecules like proteins and DNA. This property is due to the strength of the carbon-carbon bond, which allows carbon atoms to bond with each other in a variety of ways. While some of the other elements listed, such as nitrogen and chlorine, can also form bonds with themselves, they are generally less able to form long chains or rings compared to carbon.
Vraag 29 Verslag
A major factor coonsidered in selecting a suitable method for preparing a simple salt is its
Antwoorddetails
One major factor that is considered when selecting a suitable method for preparing a simple salt is its solubility in water. This is because the salt needs to dissolve in water in order to form a solution that can be used for various purposes, such as in chemistry experiments or in food preparation. If the salt is not soluble in water, then it may not be suitable for these applications. Other factors, such as its crystalline form, melting point, and reactivity with dilute acids, may also be important considerations depending on the specific application or use of the salt, but solubility in water is a key factor that must be considered in almost all cases.
Vraag 30 Verslag
Steam changes the colour of anhydrous cobalt (II) chloride from
Antwoorddetails
Anhydrous cobalt (II) chloride is blue in color, but when steam is passed over it, the blue color disappears and the compound turns pink. Therefore, the answer is "blue to pink". This is due to the fact that the cobalt (II) chloride reacts with the water in the steam to form pink hexahydrate cobalt (II) chloride.
Vraag 31 Verslag
If the relative molecular mass of an element is not a whole number, it can be deduced that the element is
Antwoorddetails
If the relative molecular mass of an element is not a whole number, it can be deduced that the element is an isotopic mixture. This is because the relative molecular mass of an element is calculated by taking the weighted average of the masses of its isotopes, where the weighting factor is the abundance of each isotope in nature. If an element has isotopes with different masses and abundances, then the weighted average will not be a whole number. Therefore, the element must be an isotopic mixture.
Vraag 32 Verslag
What amount of mercury would be liberated if the same quantity of electricity that liberates 0.65g of zinc is applied?
Antwoorddetails
Zn2+ + 2e- → Zn
2F will be liberated 65g of Zn, if 0.65gm of Zn is liberated, the quantity of electricity used
X = (0.65/65) * 2
= 0.02F
∴ Hg+ + e- → Hg1
i.e 1F → 201gm of Hg
∴ 0.02 will liberated
Y = (0.02/1) * 201
= 4.02g
Vraag 33 Verslag
Which of the following compounds can be represented by the molecular formula C2H6O?
Antwoorddetails
The molecular formula C2H6O represents a compound that contains two carbon atoms, six hydrogen atoms, and one oxygen atom. To determine which of the given compounds can be represented by this formula, we need to look at their molecular formulas and see if they match. - Propane has the molecular formula C3H8, which does not match C2H6O. Therefore, it cannot be represented by this formula. - Ethanol has the molecular formula C2H5OH, which contains two carbon atoms, six hydrogen atoms, and one oxygen atom. Therefore, it can be represented by the formula C2H6O. - Methanoic acid has the molecular formula HCOOH, which contains one carbon atom, two oxygen atoms, and two hydrogen atoms. Therefore, it cannot be represented by the formula C2H6O. - Glucose has the molecular formula C6H12O6, which does not match C2H6O. Therefore, it cannot be represented by this formula. Therefore, the compound that can be represented by the molecular formula C2H6O is Ethanol.
Vraag 34 Verslag
Which of the following ions require the largest quantity of electricity for discharge at an electrode?
Vraag 35 Verslag
which of the following polymers is suitable for packaging and electrical insulation?
Antwoorddetails
Vraag 36 Verslag
When a salt loses its water of crystallization to the atmosphere on exposure, the process is said to be
Antwoorddetails
The process of a salt losing its water of crystallization to the atmosphere on exposure is known as efflorescence. This happens when a hydrate salt is exposed to a dry atmosphere, causing the water molecules in the crystal lattice to evaporate, leaving behind the anhydrous salt. This process can be reversed by adding water to the anhydrous salt to form the hydrate again. The other options are not relevant to this process: deliquescence refers to a process where a substance absorbs water from the atmosphere and dissolves in it; effervescence refers to the escape of gas from a liquid or a solid due to a chemical reaction, and fluorescence is the emission of light by a substance that has absorbed light or other electromagnetic radiation.
Vraag 37 Verslag
A red precipitate of copper (I) carbide is formed when ammonium solution of copper (I) chloride is introduced into
Antwoorddetails
Vraag 38 Verslag
The solubility of a salt of molar mass 101g at 20oC is 0.34 mol dm-3. If 3.40g of the salt is dissolved completely in 250cm3 of water in a beaker, the resulting solution is
Antwoorddetails
To determine whether the resulting solution is a suspension, saturated, unsaturated, or supersaturated, we need to compare the amount of salt dissolved in water with its maximum solubility at that temperature. First, we need to calculate the maximum amount of salt that can dissolve in 250 cm³ of water at 20°C using the given solubility: 0.34 mol dm⁻³ = 0.34 mol / 1000 cm³ Maximum amount of salt that can dissolve in 250 cm³ of water = 0.34 x 250 / 1000 = 0.085 mol Next, we need to compare the maximum amount of salt that can dissolve in water with the actual amount of salt dissolved in 250 cm³ of water: Amount of salt dissolved in 250 cm³ of water = 3.40 g / 101 g/mol = 0.0337 mol Since the actual amount of salt dissolved in water (0.0337 mol) is less than the maximum amount of salt that can dissolve in water (0.085 mol), the resulting solution is unsaturated. Therefore, the answer is Unsaturated.
Vraag 39 Verslag
When H2S is passed into a solution of iron (III) chloride, the solution turns
Antwoorddetails
When H2S is passed into a solution of iron (III) chloride, the solution turns brown. This is due to the formation of a precipitate of iron (III) sulfide (Fe2S3). The reaction is represented as follows: 2FeCl3(aq) + 3H2S(g) → Fe2S3(s) + 6HCl(aq) The brown precipitate of Fe2S3 indicates the presence of iron (III) ions in the solution. Therefore, the correct answer is "Brown".
Vraag 40 Verslag
A suitable reagent for distinguishing between ethanoic acid ethanol is
Antwoorddetails
Vraag 41 Verslag
Which of the following chloride would exhibit the least ionic character?
Antwoorddetails
Vraag 42 Verslag
The chromatography separation of ink is based on the ability of the components to
Antwoorddetails
The chromatography separation of ink is based on the ability of the components to move at different speeds in the column. Chromatography is a technique for separating mixtures based on the differences in their physical and/or chemical properties. In ink chromatography, the mixture of colored components in the ink is separated by passing it through a stationary phase (paper or column) and a mobile phase (solvent). The colored components in the ink mixture have different degrees of solubility in the mobile phase and different affinities for the stationary phase. As a result, they will move through the stationary phase at different rates, leading to their separation. The more soluble and less attracted to the stationary phase a component is, the faster it will move through the column. Conversely, the less soluble and more attracted to the stationary phase a component is, the slower it will move. Therefore, ink chromatography separates the components of the ink mixture based on their relative speeds of movement.
Vraag 43 Verslag
The boiling of fat and aqueous caustic soda is referred to as
Antwoorddetails
The boiling of fat and aqueous caustic soda is referred to as saponification. Saponification is a process in which fat or oil is mixed with a strong base, usually sodium hydroxide (caustic soda), to produce soap. During saponification, the ester bonds of the fat molecules are broken down by the strong base, releasing the fatty acid and glycerol molecules. The fatty acid then reacts with the base to form the soap molecule, which is a salt of the fatty acid. In simpler terms, saponification is a chemical reaction that turns fat into soap by boiling it with a strong base like caustic soda. This process is used to make soap in large quantities for commercial and industrial purposes.
Vraag 44 Verslag
The product of the electrolysis of dilute sodium hydroxide using platinum electrodes are
Antwoorddetails
Vraag 45 Verslag
The intermediate product formed when ethanol is progressively oxidized to ethanoic acid with potassium heptaoxodichromate (IV) is
Antwoorddetails
When ethanol is progressively oxidized to ethanoic acid with potassium heptaoxodichromate (IV), the intermediate product formed is ethanal. This is because ethanol is first oxidized to acetaldehyde (ethanal) before further oxidation to ethanoic acid. Acetaldehyde is an intermediate product in the oxidation of ethanol to ethanoic acid. Therefore, the correct option is "ethanal".
Vraag 46 Verslag
Which of the following types of alkanols undergo oxidation to produce alkanoic acids? 1. Primary alkanols 11. Secondary alkanols 111. Tertiary alkanols
Vraag 47 Verslag
The number of isomers formed by C6H14 is
Antwoorddetails
The number of isomers formed by C6H14 is 5. Isomers are molecules with the same molecular formula but different structural arrangements of their atoms. C6H14 can form five isomers: n-hexane, 2-methylpentane, 3-methylpentane, 2,2-dimethylbutane, and 2,3-dimethylbutane. The first three are straight-chain isomers, while the last two are branched-chain isomers. These isomers have different physical and chemical properties, such as boiling points, melting points, and reactivities, which make them useful in various industrial applications.
Vraag 48 Verslag
A little quantity of trichloromethane (b.pt 60,oC) was added to a large quantity of ethanol (b.pt 78,oC). The most probable boiling point of the resultant mixture is from
Antwoorddetails
Vraag 49 Verslag
Which of the following equations show that a reaction is in equilibrium?
Antwoorddetails
When a chemical reaction reaches equilibrium, the rates of the forward and reverse reactions become equal, and the concentrations of reactants and products remain constant. At equilibrium, the Gibbs free energy change (ΔG) is equal to zero, which means that there is no net energy change during the reaction. Therefore, the equation that shows that a reaction is in equilibrium is ΔG = 0.
Vraag 50 Verslag
The gas that gives brown colouration in brown ring test is
Antwoorddetails
The gas that gives brown coloration in the brown ring test is Nitric Oxide (NO). When FeSO4 is added to a solution containing nitrate ions and concentrated H2SO4 is carefully added down the side of the test tube, a brown ring appears at the junction of the two liquids, indicating the presence of nitrate ions. This is because NO reacts with iron(II) to form a complex ion, [Fe(NO)(H2O)5]2+, which gives the brown coloration at the interface of the two layers. The other options in this question, CO and CO2, do not produce a brown ring in this test.
