MHT CET 2020 Chemistry Question Paper with Answer and Solution

(A) Pyrophosphoric acid is formed by the condensation of two molecules of orthophosphoric acid $(H_3PO_4)$ with the elimination of one water molecule $(H_2O)$.
$2H_3PO_4 \rightarrow H_4P_2O_7 + H_2O$.
As shown in the structure,there are $4$ hydrogen atoms,$2$ phosphorus atoms,and $7$ oxygen atoms.
Therefore,the molecular formula is $H_4P_2O_7$.

(D) The tendency to form an oxide is related to the standard Gibbs free energy of formation $({\Delta}G_f^{\circ})$ of the oxide. Elements with a less negative or positive ${\Delta}G_f^{\circ}$ have a lower tendency to form oxides. Among the given elements,$Hg$ (mercury) is a noble metal with a very low affinity for oxygen,making it the least likely to form a stable oxide under standard conditions.

(A) To identify the oxyacid with an $S-S$ linkage,let us examine the structures:
$1$. $H_{2}SO_{3}$ (Sulphurous acid): Structure is $(HO)_{2}S=O$. No $S-S$ bond.
$2$. $H_{2}S_{2}O_{4}$ (Dithionous acid): Structure is $(HO)S(=O)-S(=O)(OH)$. This contains an $S-S$ linkage.
$3$. $H_{2}S_{2}O_{5}$ (Disulphurous acid): Structure is $(HO)S(=O)-O-S(=O)_{2}(OH)$. This contains an $S-O-S$ linkage.
$4$. $H_{2}S_{2}O_{2}$ (Thiosulphurous acid): Structure is $H-S(=O)-S-H$. This contains an $S-S$ linkage,but $H_{2}S_{2}O_{4}$ is the standard oxyacid commonly cited for this property in textbooks.
Therefore,$H_{2}S_{2}O_{4}$ is the correct answer.

(D) The structure of thiosulphuric acid,$H_2S_2O_3$,is $HO-S(=O)(=S)-OH$.
It contains one $S=S$ bond and one $S=O$ bond.
Therefore,the correct option is $D$.

(B) When concentrated $H_2SO_4$ reacts with $PCl_5$,it undergoes a chlorination reaction to produce sulphuryl chloride $(SO_2Cl_2)$ along with phosphorus oxychloride $(POCl_3)$ and hydrogen chloride $(HCl)$.
The balanced chemical equation is:
$H_2SO_4 + 2 PCl_5 \longrightarrow SO_2Cl_2 + 2 POCl_3 + 2 HCl$

(C) Cinnabar $(HgS)$ is a mineral of sulphur and mercury.
It is the most common ore of mercury.
Fluorapatite is a phosphate mineral,Carnalite is a potassium magnesium chloride mineral,and Sylvine is a potassium chloride mineral.

(C) The structure of $H_{2}S_{2}O_{6}$ (dithionic acid) is $HO-SO_{2}-SO_{2}-OH$.
In this structure,each sulphur atom is bonded to two oxygen atoms via double bonds $(S=O)$.
Therefore,there are a total of four $S=O$ bonds in $H_{2}S_{2}O_{6}$.
The structure is as follows:
$O=S(OH)(=O)-S(OH)(=O)=O$.

(D) Marshall's acid is also known as peroxydisulfuric acid.
Its chemical structure consists of two sulfur atoms linked by a peroxide bridge $(-O-O-)$.
The molecular formula of Marshall's acid is $H_{2}S_{2}O_{8}$.
Comparing this with the given options,option $D$ is correct.

(D) The reaction of acidified potassium dichromate with $SO_{2}$ is as follows:
$K_{2}Cr_{2}O_{7} + H_{2}SO_{4} + 3SO_{2} \longrightarrow K_{2}SO_{4} + Cr_{2}(SO_{4})_{3} + H_{2}O$
In this reaction,$SO_{2}$ acts as a reducing agent and is oxidized to sulphate ions,while the orange-colored dichromate ion $(Cr_{2}O_{7}^{2-})$ is reduced to green-colored chromic ion $(Cr^{3+})$ in the form of chromic sulphate $(Cr_{2}(SO_{4})_{3})$.
Therefore,the solution turns green.

(A) The thermal stability of oxyacids of chlorine increases with an increase in the oxidation state of the chlorine atom.
The oxidation states of chlorine in the given oxyacids are:
$HClO$ $(+1)$,$HClO_{2}$ $(+3)$,$HClO_{3}$ $(+5)$,and $HClO_{4}$ $(+7)$.
Therefore,the order of thermal stability is $HClO < HClO_{2} < HClO_{3} < HClO_{4}$.
Thus,$HClO_{4}$ has the highest thermal stability.

(D) $I$. Haber process: $NH_3$ production.
$II$. Ostwald's process: $HNO_3$ production.
$III$. Deacon process: $Cl_2$ production by oxidation of $HCl$ gas.
$IV$. Contact process: $H_2SO_4$ production.

(B) The reactivity of halogens towards dihydrogen decreases down the group. Fluorine is the most reactive halogen and reacts with dihydrogen even in the dark at very low temperatures. Therefore,$F_2$ combines with $H_2$ at the lowest temperature.

(A) $Cl_{2}$ acts as a powerful bleaching and oxidising agent due to the formation of nascent oxygen in the presence of moisture.
The reaction is: $Cl_{2} + H_{2}O \rightarrow HCl + HOCl$
$HOCl \rightarrow HCl + [O]$
This nascent oxygen $[O]$ is responsible for the bleaching action of $Cl_{2}$.

(D) Fluorine $(F)$ is the most electronegative element and has a small atomic size.
It lacks $d$-orbitals in its valence shell,which prevents it from expanding its octet or forming polyhalide ions like $I_3^-$ or $Br_3^-$.
Therefore,$F$ does not form polyhalide ions.

(B) $Cl$ occurs commonly in both the earth's crust and seawater.
The most common compound of chlorine in seawater is $NaCl$,and a small amount of $KCl$ is also present.
The common minerals of $Cl$ are Rocksalt $(NaCl)$,Sylvine $(KCl)$,and Carnallite $(KCl \cdot MgCl_2 \cdot 6H_2O)$.
Cryolite $(Na_3AlF_6)$ is a mineral of fluorine,not chlorine.

(C) To determine the number of lone pairs on the chlorine atom,we examine the oxidation state and bonding of chlorine in each oxyacid:
$1$. $HOCl$ $(HClO)$: Chlorine is in the $+1$ oxidation state. It forms $1$ single bond with $O$ and has $3$ lone pairs.
$2$. $HOClO$ $(HClO_{2})$: Chlorine is in the $+3$ oxidation state. It forms $1$ single bond with $OH$ and $1$ double bond with $O$,leaving $2$ lone pairs.
$3$. $HOClO_{2}$ $(HClO_{3})$: Chlorine is in the $+5$ oxidation state. It forms $1$ single bond with $OH$ and $2$ double bonds with $O$,leaving $1$ lone pair.
$4$. $HOClO_{3}$ $(HClO_{4})$: Chlorine is in the $+7$ oxidation state. It forms $1$ single bond with $OH$ and $3$ double bonds with $O$. Since chlorine has $7$ valence electrons and all are involved in bonding,there are $0$ lone pairs on the chlorine atom.
Therefore,$HOClO_{3}$ is the correct answer.

(A) The oxidizing power of oxoacids of chlorine depends on the oxidation state of the central chlorine atom.
Lower oxidation state of the central atom corresponds to higher oxidizing power.
The oxidation states of $Cl$ in $HClO$,$HClO_{2}$,and $HClO_{3}$ are $+1$,$+3$,and $+5$ respectively.
Therefore,the decreasing order of oxidizing power is $HClO > HClO_{2} > HClO_{3}$.

(C) The correct answer is $I$ and $F$.
Interhalogen compounds of the type $XX_{7}^{\prime}$ are formed when a large halogen atom $(X)$ reacts with a smaller,highly electronegative halogen atom $(X^{\prime})$.
Iodine heptafluoride $(IF_{7})$ is the only known interhalogen compound of this type.
It is formed by the reaction of iodine $(I)$ with fluorine $(F)$.
According to $VSEPR$ theory,$IF_{7}$ has a pentagonal bipyramidal structure.

(B) $Cl_2$ (Chlorine) is a common bleaching agent used in the paper industry to whiten wood pulp.
It acts as an oxidizing agent to remove lignin,which gives wood its brown color.
While chlorine dioxide $(ClO_2)$ is often preferred in modern $ECF$ (Elemental Chlorine Free) bleaching processes to reduce the formation of organochlorides,$Cl_2$ is historically and industrially recognized as the primary bleaching agent for this purpose.

(D) Radon $(Rn)$ is a radioactive element.
Due to its radioactive nature,it is used in radiotherapy for the treatment of cancer.

(B) Krypton $(Kr)$ is the noble gas used in miner's cap lamps because it provides a bright,white light and has a long filament life.

(B) The inert gases $Xe$ (Xenon) and $Kr$ (Krypton) are commonly used in high-intensity flash bulbs for photography because they produce a bright white light when an electric discharge is passed through them.

(C) $Helium$ $(He)$ is the inert gas used for filling balloons because it is much lighter than air and is chemically inert,making it safe to use.

(D) Terpenes are unsaturated hydrocarbons that are classified by the number of isoprene units $(C_5H_8)$,with the most common being mono-,sesqui-,and di-terpenes ($C_{10}$,$C_{15}$,and $C_{20}$,respectively).
Terpenes are highly aromatic compounds that determine the smell of many plants and herbs,such as rosemary and lavender,as well as some animals.
The side chains of Vitamin $A$,$E$,$K$,and squalene are all constituents of terpenes.
Terpenes are important constituents of essential oils,having a chemical structure consisting of repeated isoprene $(C_5H_8)$ units. Since they contain double bonds,they are unsaturated,not saturated.

(C) $i$. Linear polymer: $Polyvinylchloride$ $(PVC)$ consists of long,straight chains of monomers.
$ii$. Branched chain polymer: $Polypropylene$ contains branches along the main chain.
$iii$. Network or cross-linked polymers: $Vulcanised \text{ } rubber$ and $Melamine$ contain cross-links between polymer chains.

(A) $PHBV$ (Poly-$\beta$-hydroxybutyrate-co-$\beta$-hydroxyvalerate) is a biodegradable aliphatic polyester.
It is obtained by the copolymerisation of $3-$hydroxybutanoic acid and $3-$hydroxypentanoic acid.
The reaction is:
$n CH_3-CH(OH)-CH_2-COOH n CH_3-CH_2-CH(OH)-CH_2-COOH \rightarrow -[O-CH(CH_3)-CH_2-CO-O-CH(CH_2CH_3)-CH_2-CO]_n-$

(A) Melamine is a thermosetting polymer formed by the condensation polymerization of melamine and formaldehyde.
Due to its high heat resistance and hardness,it is widely used in the manufacturing of unbreakable crockeries and kitchenware.

(B) Thermoplastic polymers are linear or branched chain molecules that can be repeatedly softened on heating and hardened on cooling.
Because of this property,they can be remoulded and recycled.
Therefore,the correct property is that they are either linear or branched chain polymers.

(C) Elastomers are polymers in which the polymer chains are held together by the weakest intermolecular forces. These weak forces permit the polymer to be stretched.
$Neoprene$ (polychloroprene) is a synthetic rubber and belongs to the class of elastomers.
$Polystyrene$ is a thermoplastic,$Nylon$ is a fiber,and $Bakelite$ is a thermosetting polymer.

(C) Biodegradable polymers are those that can be decomposed by microorganisms.
$Dextron$ (also known as $PHBV$ or poly-$\beta$-hydroxybutyrate-co-$\beta$-hydroxyvalerate) is a well-known biodegradable polymer.
$PVC$,$Polythene$,and $Teflon$ are synthetic,non-biodegradable polymers.

(A) Thermosetting polymers are heavily cross-linked polymers.
When heated,they undergo extensive cross-linking in molds and become infusible and insoluble.
Therefore,they cannot be softened or remoulded by heating,which makes them non-recyclable.
Thus,the correct property is that they are cross-linked polymers.

(B) Thermoplastic polymers are linear or slightly branched long-chain molecules that can be repeatedly softened on heating and hardened on cooling.
$Polythene$ is a classic example of a thermoplastic polymer.
$Bakelite$ is a thermosetting polymer.
$Terylene$ is a polyester fiber (often considered a thermoplastic,but $Polythene$ is the most standard textbook example for this classification).
$Neoprene$ is a synthetic rubber (elastomer).

(C) Thermoplastic polymers are linear or slightly branched long-chain molecules capable of repeatedly softening on heating and hardening on cooling.
$Polystyrene$ is a well-known thermoplastic polymer.
$Bakelite$ is a thermosetting polymer.
$Nylon-6$ is a fiber (polyamide).
$Neoprene$ is an elastomer.
Therefore,the correct answer is $Polystyrene$.

(A) Poly$(3-hydroxybutyrate-co-3-hydroxyvalerate)$ $(PHBV)$ is a biodegradable polymer.
It is obtained by the copolymerization of $3-hydroxybutanoic$ acid and $3-hydroxypentanoic$ acid.
$Buna-N$,$PTFE$,and $PVC$ are synthetic,non-biodegradable polymers.

(C) Neoprene is a synthetic rubber produced by the free radical polymerization of chloroprene.
The chemical name for chloroprene is $2-$chlorobuta$-1,3-$diene.
The reaction is as follows:
$n \ CH_2=C(Cl)-CH=CH_2 \xrightarrow{\text{polymerization}} [-CH_2-C(Cl)=CH-CH_2-]_n$

(A) The copolymerization of isobutylene ($2$-methylpropene) with a small amount of isoprene ($2$-methyl-$1,3$-butadiene) in the presence of anhydrous $AlCl_{3}$ at low temperatures (around $-100^{\circ} C$) yields Butyl rubber.
The chemical reaction is as follows:
$n \ CH_{2}=C(CH_{3})_{2} + n \ CH_{2}=C(CH_{3})-CH=CH_{2}$ $\xrightarrow{AlCl_{3}} -[CH_{2}-C(CH_{3})_{2}-CH_{2}-C(CH_{3})=CH-CH_{2}]_{n}-$
Thus,the correct option is $A$.

(C) Orlon (polyacrylonitrile) is formed by the addition polymerization of acrylonitrile monomers.
Nylon-$6, 6$,polyester,and urea-formaldehyde polymers are formed by condensation polymerization,which involves the elimination of small molecules like $H_2O$ or $CH_3OH$.

(A) Bakelite is a thermosetting polymer prepared by the condensation polymerization of phenol and formaldehyde in the presence of an acid or base catalyst.
Therefore,the correct option is $A$.

(B) Teflon is manufactured by heating tetrafluoroethene with a free radical or persulphate catalyst at high pressures.
The chemical reaction is:
$n CF_2=CF_2 \xrightarrow{\text{Catalyst, High Pressure}} -(CF_2-CF_2)-_n$
Thus,the monomer used to prepare Teflon is tetrafluoroethene $(CF_2=CF_2)$.

(C) Cross-linked polymers are formed from bi-functional and tri-functional monomers and contain strong covalent bonds between various linear polymer chains.
$PVC$,$Orlon$,and $Polypropylene$ are examples of linear or thermoplastic polymers.
$Vulcanised$ $rubber$ is a classic example of a cross-linked polymer,where sulfur bridges create cross-links between the polyisoprene chains,making the material harder and more durable.

(C) The $-CO-NH-$ linkage is known as the amide linkage or peptide linkage.
$Caprolactam$ is a cyclic amide which,upon ring-opening polymerization,forms $Nylon-6$. The structure of $Caprolactam$ contains the $-CO-NH-$ group within its ring.
Vinyl cyanide $(CH_2=CH-CN)$ contains a nitrile group.
Dimethyl terephthalate contains ester linkages $(-COO-)$.
Hexamethylenediamine $(H_2N-(CH_2)_6-NH_2)$ contains amine groups.

(A) $PHBV$ stands for Poly $\beta$-hydroxybutyrate-co-$\beta$-hydroxyvalerate.
It is a copolymer obtained by the polymerization of $3-$hydroxybutanoic acid and $3-$hydroxypentanoic acid.
These monomers undergo condensation polymerization to form the biodegradable polyester $PHBV$.

(D) Natural rubber is a linear polymer of isoprene ($2-$methyl$-1,3-$butadiene).
It is formed by the polymerization of isoprene units.
The reaction is as follows:
$n \ CH_2=C(CH_3)-CH=CH_2 \xrightarrow{\text{Polymerization}} [CH_2-C(CH_3)=CH-CH_2]_n$
Thus,the monomer of natural rubber is $2-$methyl$-1,3-$butadiene.

(D) $Dacron$ (also known as $Terylene$) and $PHBV$ $(Poly-\beta-hydroxybutyrate-co-\beta-hydroxyvalerate)$ are polyesters,meaning they contain ester linkages $(-COO-)$.
$Nylon-2-nylon-6$ is a polyamide copolymer formed from glycine and amino caproic acid,which contains amide linkages $(-CONH-)$ instead of ester linkages.

(C) High density polythene $(HDPE)$ is manufactured by the addition polymerisation of ethene in the presence of a Ziegler-Natta catalyst.
The Ziegler-Natta catalyst consists of a mixture of triethylaluminium $(Al(C_{2}H_{5})_{3})$ and titanium tetrachloride $(TiCl_{4})$.

(D) Buna-$S$ or styrene butadiene rubber $(SBR)$ is a copolymer formed by the polymerization of styrene $(C_6H_5CH=CH_2)$ and $1,3-$butadiene $(CH_2=CH-CH=CH_2)$.
The reaction is as follows:
$n(C_6H_5CH=CH_2) + n(CH_2=CH-CH=CH_2) \xrightarrow{Peroxide} [-CH(C_6H_5)-CH_2-CH_2-CH=CH-CH_2-]_n$
Thus,the correct answer is $SBR$.

(B) $PVC$ (Polyvinyl chloride) is widely used in the manufacturing of synthetic leather,often referred to as artificial leather or rexine. It is durable,water-resistant,and cost-effective compared to natural leather.

(A) Lycopene,$\beta$-carotene,and lutein are natural carotenoids classified as tetraterpenes.
These compounds are composed of eight isoprene units.

(C) Novolac is a linear polymer formed by the condensation polymerization of $Phenol$ and $Methanal$ (formaldehyde) in the presence of an acid catalyst.
The reaction involves the initial formation of $o$- or $p$-hydroxybenzyl alcohol derivatives,which then undergo condensation to form a linear chain structure known as Novolac.
Therefore,the correct monomers are $Phenol$ and $Methanal$.

(B) Bakelite is a thermosetting polymer formed by the condensation reaction of phenol and formaldehyde.
It is a poor conductor of heat and electricity,which makes it ideal for manufacturing electrical switches,plugs,and handles of cooking utensils like cookers.