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Punjab Chemistry 2018 Paper 1 — Solved Past Paper with Answers
All 17 MCQs from Punjab Chemistry 2018 Paper 1, solved with the correct answer highlighted and a full explanation for every question. This is a free MDCAT Punjab / UHS past paper — no signup, no ads. Practise it interactively in timed mode, drill more with free MDCAT MCQs, or browse all Punjab / UHS papers.
Attempt this paper interactively →Q1. The rate of E1 reaction depends upon:
- A. The concentration of substrate✓
- B. The concentration of nucleophile
- C. The concentration of substrate as well as nucleophile
- D. None of these
Explanation: In E1 (unimolecular elimination) reactions, the rate-determining step involves the formation of a carbocation intermediate through the dissociation of the leaving group from the substrate molecule. Therefore, the concentration of the substrate directly affects the rate of the reaction because it determines the frequency of collisions between the substrate molecules and the solvent molecules, which leads to the formation of the carbocation intermediate.
Why the other options are wrong
- B. In E1 reactions, the nucleophile is typically not involved in the rate-determining step. The reaction proceeds via the formation of a carbocation intermediate, followed by the attack of the nucleophile on the carbocation to form the product. Since the nucleophile does not directly participate in the rate-determining step, its concentration does not significantly affect the rate of the reaction.
- C. While the concentration of the substrate affects the rate of the E1 reaction, the concentration of the nucleophile usually does not. Therefore, option C is not correct because it suggests that both the substrate and nucleophile concentrations influence the rate, which is not typically the case in E1 reactions.
- D. This option implies that neither the concentration of the substrate nor the concentration of the nucleophile affects the rate of the E1 reaction. However, as explained earlier, the concentration of the substrate does affect the rate of the reaction in E1 mechanisms.
Q2. Which compound is more soluble in water?
- A. C2H5OH✓
- B. C6H5OH
- C. CH3COCH3
- D. n-Hexanol
Explanation: Ethanol is highly soluble in water because it can form hydrogen bonds with water molecules through its hydroxyl (-OH) group. This allows ethanol molecules to mix well with water molecules, leading to high solubility.
Why the other options are wrong
- B. Phenol is also soluble in water, but to a lesser extent compared to ethanol. It can form hydrogen bonds with water molecules through its hydroxyl (-OH) group, but the bulky phenyl group (-C6H5) reduces its overall solubility compared to ethanol.
- C. Acetone is a polar solvent and is miscible with water, meaning it can dissolve in water in all proportions. Its carbonyl group (-C=O) allows it to form hydrogen bonds with water molecules, increasing its solubility.
- D. n-Hexanol is less soluble in water compared to the other options. While it has a hydroxyl (-OH) group like ethanol and phenol, the longer carbon chain reduces its polarity and ability to form hydrogen bonds with water molecules, resulting in lower solubility.
Q3. Cannizzaro's reaction is not given by:
- A. Formaldehyde
- B. Acetaldehyde
- C. Benzaldehyde
- D. Trimethyl acetaldehyde✓
Explanation: Trimethyl acetaldehyde does not have an available hydrogen atom attached to the carbonyl carbon necessary for Cannizzaro's reaction. As a result, it does not typically undergo Cannizzaro's reaction.
Why the other options are wrong
- A. Formaldehyde (HCHO) undergoes Cannizzaro's reaction readily. In this reaction, one molecule of formaldehyde is oxidized to formic acid (HCOOH), while another molecule is reduced to methanol (CH3OH).
- B. Acetaldehyde (CH3CHO) does not typically undergo Cannizzaro's reaction. Instead, it can undergo aldol condensation or oxidation to form acetic acid (CH3COOH).
- C. Benzaldehyde (C6H5CHO) can undergo Cannizzaro's reaction. It is oxidized to benzoic acid (C6H5COOH) while another molecule is reduced to benzyl alcohol (C6H5CH2OH).
Q4. Which is basic amino acid?
- A. Glycine
- B. Alanine
- C. Aspartic acid
- D. Lysine✓
Explanation: Lysine is a basic amino acid because it contains an amino group with a pKa greater than 7. At physiological pH, lysine will have a positive charge, making it basic.
Why the other options are wrong
- A. Glycine is the simplest amino acid and is not classified as either basic or acidic. It has a hydrogen atom as its side chain, making it neutral in charge.
- B. Alanine is a nonpolar, aliphatic amino acid. It is not considered basic as it does not contain an amino group that can be protonated to carry a positive charge.
- C. Aspartic acid is an acidic amino acid because it contains a carboxylic acid group, which can donate a proton and become negatively charged at physiological pH.
Q5. Which one of the following nitrogeneous bases is not present in RNA?
- A. Cytosine
- B. Adenine
- C. Thymine✓
- D. Uracil
Explanation: Thymine is a nitrogenous base found only in DNA. It pairs with adenine via two hydrogen bonds, forming one of the complementary base pairs in the DNA double helix.
Why the other options are wrong
- A. Cytosine is one of the four nitrogenous bases found in both RNA and DNA. It pairs with guanine via three hydrogen bonds in DNA and RNA.
- B. Adenine is another nitrogenous base found in both RNA and DNA. In RNA, adenine pairs with uracil via two hydrogen bonds, while in DNA, it pairs with thymine via two hydrogen bonds.
- D. Uracil is a nitrogenous base found in RNA, replacing thymine. In RNA, uracil pairs with adenine via two hydrogen bonds, forming one of the complementary base pairs.
Q6. Micronutrients are required in quantity ranging from:
- A. 4-40 g✓
- B. 6-200 g
- C. 6-200 kg
- D. 4-40 kg
Explanation: Micronutrients are required in very small quantities, typically measured in grams (g). This option represents the typical range of intake for micronutrients, which is usually between 4 to 40 grams.
Why the other options are wrong
- B. This option suggests a wider range of intake, ranging from 6 to 200 grams. However, this range is too high for micronutrients, which are required in much smaller amounts.
- C. This option represents an extremely high quantity, measured in kilograms (kg). Micronutrients are needed in much smaller amounts than this, making this option incorrect.
- D. Similar to option C, this suggests a very high quantity, measured in kilograms. However, it's still too high for micronutrients, which are required in much smaller amounts.
Q7. The pH range of acid rain is:
- A. 7-6.5
- B. 6-5.6
- C. Less than 5✓
- D. 6.5-6
Explanation: This option (less than 5) represents the typical pH range of acid rain. Acid rain is defined as any precipitation with a pH below 5.0, indicating acidic conditions.
Why the other options are wrong
- A. This range indicates a slightly acidic to neutral pH, which is not characteristic of acid rain. Acid rain typically has a pH below neutral.
- B. This range also suggests slightly acidic conditions, but it's not within the typical pH range of acid rain, which tends to be lower.
- D. This range indicates slightly acidic conditions but is not low enough to be considered as acid rain. Acid rain typically has a pH below 5.
Q8. Which one of the following is a secondary pollutant?
- A. CO
- B. NOX
- C. SOX
- D. PAN✓
Explanation: PAN is a secondary pollutant formed in the atmosphere through the reaction of volatile organic compounds (VOCs) and nitrogen oxides (NOX) in the presence of sunlight. It is a component of photochemical smog and contributes to respiratory issues and plant damage.
Why the other options are wrong
- A. Carbon monoxide is a primary pollutant, meaning it is directly emitted into the atmosphere from sources such as vehicles, industrial processes, and combustion of fossil fuels.
- B. Nitrogen oxides, including nitric oxide (NO) and nitrogen dioxide (NO2), are primary pollutants emitted from sources like vehicles, power plants, and industrial processes. However, they can also undergo chemical reactions in the atmosphere to form secondary pollutants like ozone and nitric acid.
- C. Like nitrogen oxides, sulfur oxides, including sulfur dioxide (SO2), are primary pollutants emitted from sources such as power plants and industrial facilities. They can also undergo atmospheric reactions to form secondary pollutants like sulfuric acid and sulfate particles.
Q9. Which of the following statement is incorrect?
- A. All the metals are good conductor of heat
- B. All the metals form positive ion
- C. All the metals are good conductor of electricity
- D. All the metals form acidic oxides✓
Explanation: Metals generally form basic or amphoteric oxides rather than acidic oxides. Basic oxides react with water to form basic solutions, while amphoteric oxides can react with both acids and bases. Nonmetals, on the other hand, tend to form acidic oxides.
Why the other options are wrong
- A. This statement is generally true. Metals are typically good conductors of heat due to their crystalline structure and the mobility of their electrons, which allows heat energy to be transmitted through them efficiently.
- B. This statement is true. Metals tend to lose electrons to form positively charged ions (cations). This process occurs when metals undergo chemical reactions to achieve a stable electron configuration, usually by achieving the electron configuration of the nearest noble gas.
- C. This statement is generally true. Similar to heat conduction, metals are good conductors of electricity due to the mobility of their electrons. The delocalized electrons in metals are free to move throughout the material, allowing electric current to flow easily.
Q10. Which of the following is not an alkali metal?
- A. Francium
- B. Cesium
- C. Rubidium
- D. Radium✓
Explanation: Radium is not an alkali metal; it is an alkaline earth metal. It is a highly radioactive element that is silvery-white in color. Radium was formerly used in luminous paints for watches, but due to its radioactive properties, its use has been largely discontinued.
Why the other options are wrong
- A. Francium is an alkali metal and is the least stable of the first 101 elements of the periodic table. It is a highly radioactive metal and is extremely rare, occurring naturally only in trace amounts due to its short half-life.
- B. Cesium is also an alkali metal. It is a soft, silvery-gold alkali metal with a melting point just above room temperature. Cesium is used in atomic clocks, as well as in the oil industry to measure well depths.
- C. Like francium and cesium, rubidium is an alkali metal. It is a soft, silvery-white metallic element that is highly reactive and is used in various applications such as in atomic clocks, photocells, and as a component of some special glasses.
Q11. Tincal is a mineral of:
- A. Al
- B. B✓
- C. Si
- D. C
Explanation: Borax is the sodium salt of tetraboric acid. It is the most important of all borates .Borax occurs as a natural deposit called tincal in the dried up lakes of Tibet and California.Borax may also be obtained from tincal (Na2B4O7.10H2O) by treating tincalwith water and subsequently evaporating the clear solution, when crystalsof borax separate out.
Why the other options are wrong
- A. It is not a mineral of Al
- C. It is not a mineral of Si
- D. It is not a mineral of carbon
Q12. The brown gas formed, when metal reduces HNO3 to:
- A. NO
- B. NO
- C. NO2✓
- D. N2O4
Explanation: This represents nitrogen dioxide, which is a brown gas commonly formed when a metal reduces nitric acid (HNO₃). Nitrogen dioxide is a prominent air pollutant and is often produced in industrial processes involving nitrogen and oxygen.
Why the other options are wrong
- A. This represents nitric oxide, which can be formed in reactions involving nitrogen and oxygen. However, when a metal reduces nitric acid, the resulting gas is typically brown due to the formation of nitrogen dioxide (NO₂), not nitric oxide.
- B. This represents nitric oxide, which can be formed in reactions involving nitrogen and oxygen. However, when a metal reduces nitric acid, the resulting gas is typically brown due to the formation of nitrogen dioxide (NO₂), not nitric oxide.
- D. This represents dinitrogen tetroxide, which is a colorless gas at room temperature. It can appear brownish when it undergoes partial decomposition or when mixed with other substances, but it's not typically the primary brown gas formed when a metal reduces nitric acid.
Q13. Which halogen occurs naturally in a positive oxidation state?
- A. F
- B. Cl
- C. Br
- D. I✓
Explanation: Iodine occurs naturally in positive oxidation states more frequently than bromine. Iodine can exist in compounds such as iodine(I) chloride (ICl) or iodine(I) fluoride (IF), where it also has an oxidation state of +1. Therefore, iodine is the halogen that occurs naturally in a positive oxidation state.
Why the other options are wrong
- A. Fluorine is highly electronegative and typically occurs in compounds with a negative oxidation state (-1), such as fluoride ions (F⁻). It does not naturally occur in a positive oxidation state.
- B. Chlorine, like fluorine, is also highly electronegative and typically exists in compounds with a negative oxidation state (-1), such as chloride ions (Cl⁻). It does not naturally occur in a positive oxidation state.
- C. Bromine is known to occur naturally in positive oxidation states, particularly in compounds like bromine(I) chloride (BrCl) or bromine(I) fluoride (BrF), where it has an oxidation state of +1. However, there's another halogen that occurs more prominently in positive oxidation states.
Q14. Which of the following is a non-typical transition element?
- A. Cr
- B. Mn
- C. Zn✓
- D. Fe
Explanation: Zinc is not considered a transition element. While it is classified as a metal, it does not exhibit the typical properties of transition metals. Zinc has a full d shell in its electron configuration and does not form colored ions or complexes.
Why the other options are wrong
- A. Chromium is a typical transition element commonly found in the transition metal block of the periodic table. It exhibits typical transition metal properties such as variable oxidation states and the formation of colored compounds.
- B. Manganese is a non-typical transition element because it has a broader range of oxidation states compared to most transition metals. While most transition metals have multiple oxidation states, manganese exhibits a wider range, including very high oxidation states such as +7 in compounds like potassium permanganate (KMnO₄).
- D. Iron is a typical transition element commonly found in the transition metal block of the periodic table. It displays typical transition metal properties such as variable oxidation states and the ability to form colored compounds.
Q15. Ethers show the phenomenon of:
- A. Metamerism✓
- B. Functional group isomerism
- C. Position isomerism
- D. Cis-trans isomerism
Explanation: Ethers show the phenomenon of metamerism. Metamerism is a type of structural isomerism where compounds have the same molecular formula but differ in the arrangement of alkyl groups attached to the oxygen atom.
Why the other options are wrong
- B. Ethers do not exhibit functional group isomerism. Functional group isomerism occurs when compounds have the same molecular formula but differ in the functional group present.
- C. Ethers also do not display position isomerism. Position isomerism occurs when compounds have the same functional groups but differ in the position of those groups within the molecule
- D. Ethers do not have double bonds, so they cannot exhibit cis-trans isomerism. Cis-trans isomerism occurs in compounds with double bonds where the groups attached to the carbons of the double bond can be arranged differently in space.
Q16. Characteristic reactions of alkenes are:
- A. Nucleophilic addition
- B. Electrophilic addition✓
- C. Nucleophilic substitution
- D. Free radical substitution
Explanation: Electrophilic addition reactions involve the addition of an electrophile (an electron-deficient species) to the π-bond of an alkene. This process is characteristic of alkenes and leads to the formation of saturated compounds. Examples include hydration, halogenation, and hydrogenation reactions.
Why the other options are wrong
- A. Nucleophilic addition involves the addition of a nucleophile (an electron-rich species) to a molecule. While nucleophilic addition reactions are common in carbonyl compounds like aldehydes and ketones, they are not characteristic reactions of alkenes. Alkenes typically undergo electrophilic addition reactions rather than nucleophilic addition.
- C. Nucleophilic substitution involves the substitution of one nucleophile for another in a molecule. This type of reaction is common in alkyl halides but is not a characteristic reaction of alkenes. Alkenes do not typically undergo nucleophilic substitution reactions.
- D. Free radical substitution reactions involve the substitution of atoms or groups in a molecule through a free radical mechanism. While some compounds, such as alkanes, can undergo free radical substitution, alkenes generally do not undergo this type of reaction as readily. Instead, they are more prone to undergo electrophilic addition reactions.
Q17. During nitration of benzene, the active nitrating agent is:
- A. NO;' +1
- B. NO; +2
- C. NO;' -1✓
- D. HNO, 3
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