9.7 (c)]. This can be represented in the form This is often referred to as Hückel Rule.of two doughtnuts (rings) of electron clouds [Fig. 9.7 (d)], one above and one below the Some examples of aromatic compounds are plane of the hexagonal ring as shown below: given below: (electron cloud) Fig. 9.7 (c) Fig. 9.7 (d) The six π electrons are thus delocalised and can move freely about the six carbon nuclei, instead of any two as shown in Fig. 9.6 (a) or (b). The delocalised π electron cloud is attracted more strongly by the nuclei of the carbon atoms than the electron cloud localised between two carbon atoms. Therefore, presence of delocalised π electrons in benzene makes it more stable than the hypothetical cyclohexatriene. X-Ray diffraction data reveals that benzene is a planar molecule. Had any one of the above structures of benzene (A or B) been correct, 9.5.4 Preparation of Benzene two types of C—C bond lengths were expected. Benzene is commercially isolated from coalHowever, X-ray data indicates that all the tar. However, it may be prepared in thesix C—C bond lengths are of the same order laboratory by the following methods.(139 pm) which is intermediate between C— C single bond (154 pm) and C—C double (i) Cyclic polymerisation of ethyne: bond (133 pm). Thus the absence of pure (Section 9.4.4) double bond in benzene accounts for the (ii) Decarboxylation of aromatic acids: reluctance of benzene to show addition Sodium salt of benzoic acid on heating reactions under normal conditions, thus with sodalime gives benzene. explaining the unusual behaviour of benzene. 9.5.3 Aromaticity Benzene was considered as parent ‘aromatic’ compound. Now, the name is applied to all the ring systems whether or not having benzene (9.70) ring, possessing following characteristics. Reprint 2025-26 322 chemistry (iii) Reduction of phenol: Phenol is reduced (ii) Halogenation: Arenes react with halogens to benzene by passing its vapours over in the presence of a Lewis acid like anhydrous heated zinc dust FeCl3, FeBr3 or AlCl3 to yield haloarenes. (9.71) Chlorobenzene 9.5.5 Properties (9.73) (iii) Sulphonation: The replacement of aPhysical properties hydrogen atom by a sulphonic acid group inAromatic hydrocarbons are non- polar a ring is called sulphonation. It is carried outmolecules and are usually colourless liquids by heating benzene with fuming sulphuricor solids with a characteristic aroma. You are acid (oleum).also familiar with naphthalene balls which are used in toilets and for preservation of clothes because of unique smell of the compound and the moth repellent property. Aromatic hydrocarbons are immiscible with water but are readily miscible with organic solvents. They burn with sooty flame. Chemical properties (9.74) Arenes are characterised by electrophilic (iv) Friedel-Crafts alkylation reaction:substitution reactions. However, under When benzene is treated with an alkyl halidespecial conditions they can also undergo in the presence of anhydrous aluminiumaddition and oxidation reactions. chloride, alkylbenene is formed. Electrophilic substitution reactions The common electrophilic substitution reactions of arenes are nitration, halogenation, sulphonation, Friedel Craft’s alkylation and acylation reactions in which attacking reagent is an electrophile (E +) (i) Nitration: A nitro group is introduced (9.75) into benzene ring when benzene is heated with a mixture of concentrated nitric acid and concentrated sulphuric acid (nitrating mixture). (9.76) Why do we get isopropyl benzene on treating benzene with 1-chloropropane instead of n-propyl benzene? (v) Friedel-Crafts acylation reaction: The reaction of benzene with an acyl halide or (9.72) acid anhydride in the presence of Lewis acids (AlCl3) yields acyl benzene. Nitrobenzene Reprint 2025-26 Hydrocarbons 323 (9.77) In the case of nitration, the electrophile, nitronium ion, is produced by transfer of a proton (from sulphuric acid) to nitric acid in the following manner: (9.78) Step I If excess of electrophilic reagent is used, further substitution reaction may take place in which other hydrogen atoms of benzene Step II ring may also be successively replaced by the electrophile. For example, benzene on treatment with excess of chlorine in the presence of anhydrous AlCl3 can be Protonated Nitronium chlorinated to hexachlorobenzene (C6Cl6) nitric acid ion It is interesting to note that in the process of generation of nitronium ion, sulphuric acid serves as an acid and nitric acid as a base. Thus, it is a simple acid-base equilibrium. (b) F o r m a t i o n o f C a r b o c a t i o n (arenium ion): Attack of electrophile results in the formation of σ-complex or (9.79) 3 arenium ion in which one of the carbon is sp Mechanism of electrophilic substitution hybridised. reactions: According to experimental evidences, SE (S = substitution; E = electrophilic) reactions are supposed to proceed via the following three steps: (a) Generation of the eletrophile sigma complex (arenium ion) (b) Formation of carbocation intermediate The arenium ion gets stabilised by resonance:(c) Removal of proton from the carbocation intermediate (a) Generation of electrophile E ⊕: During chlorination, alkylation and acylation of benzene, anhydrous AlCl3, being a Lewis acid helps in generation of the elctrophile Cl⊕, R ⊕, RC⊕O (acylium ion) respectively by combining with the attacking reagent. Reprint 2025-26 324 chemistry Sigma complex or arenium ion loses its chemical equation: aromatic character because delocalisation of 3 CxHy + (x + ) O2 → x CO2 + H2O n (9.83)electrons stops at sp hybridised carbon. (c) Removal of proton: To restore the 9.5.6 Directive influence of a functional aromatic character, σ -complex releases group in monosubstituted benzene proton from sp3 hybridised carbon on attack – When monosubstituted benzene is subjected by [AlCl4] (in case of halogenation, alkylation – to further substitution, three possible and acylation) and [HSO4] (in case of disubstituted products are not formed in nitration). equal amounts. Two types of behaviour are observed. Either ortho and para products or meta product is predominantly formed. It has also been observed that this behaviour depends on the nature of the substituent already present in the benzene ring and not on the nature of the entering group. This is known as directive influence of substituents. Reasons for ortho/para or meta directive nature of groups are discussed below: Addition reactions Ortho and para directing groups: The Under vigorous conditions, i.e., at high groups which direct the incoming group to temperature and/ or pressure in the presence ortho and para positions are called ortho and of nickel catalyst, hydrogenation of benzene para directing groups. As an example, let us gives cyclohexane. discuss the directive influence of phenolic (–OH) group. Phenol is resonance hybrid of following structures: Cyclohexane (9.80) Under ultra-violet light, three chlorine molecules add to benzene to produce benzene hexachloride, C6H6Cl6 which is also called gammaxane. Benzene hexachloride, It is clear from the above resonating (BHC) structures that the electron density is more on (9.81) o – and p – positions. Hence, the substitution Combustion: When heated in air, benzene takes place mainly at these positions. However, burns with sooty flame producing CO2 and it may be noted that –I effect of – OH group also H2O operates due to which the electron density on 15 ortho and para positions of the benzene ring C H6 + O2 → 6CO2 +3H2 O is slightly reduced. But the overall electron 6 2 (9.82) density increases at these positions of the General combustion reaction for any ring due to resonance. Therefore, –OH group hydrocarbon may be given by the following activates the benzene ring for the attack by Reprint 2025-26 Hydrocarbons 325 an electrophile. Other examples of activating In this case, the overall electron density groups are –NH2, –NHR, –NHCOCH3, –OCH3, on benzene ring decreases making further –CH3, –C2H5, etc. substitution difficult, therefore these groups are also called ‘deactivating groups’. TheIn the case of aryl halides, halogens are moderately deactivating. Because of their electron density on o – and p – position strong – I effect, overall electron density on is comparatively less than that at meta benzene ring decreases. It makes further position. Hence, the electrophile attacks on substitution difficult. However, due to comparatively electron rich meta position resonance the electron density on o– and resulting in meta substitution. p– positions is greater than that at the 9.6 Carcinogenicity and Toxicitym-position. Hence, they are also o– and p – directing groups. Resonance structures of Benzene and polynuclear hydrocarbons chlorobenzene are given below: containing more than two benzene rings fused together are toxic and said to possess cancer producing (carcinogenic) property. Such polynuclear hydrocarbons are formed on incomplete combustion of organic materials like tobacco, coal and petroleum. They enter into human body and undergo various biochemical reactions and finally damage DNA and cause cancer. Some of the carcinogenic hydrocarbons are given below (see box). Meta directing group: The groups which direct the incoming group to meta position are called meta directing groups. Some examples of meta directing groups are –NO2, –CN, –CHO, –COR, –COOH, –COOR, –SO3H, etc. Let us take the example of nitro group. Nitro group reduces the electron density in the benzene ring due to its strong–I effect. Nitrobenzene is a resonance hybrid of the following structures. Reprint 2025-26 326 chemistry SUMMARY Hydrocarbons are the compounds of carbon and hydrogen only. Hydrocarbons are mainly obtained from coal and petroleum, which are the major sources of energy. Petrochemicals are the prominent starting materials used for the manufacture of a large number of commercially important products. LPG (liquefied petroleum gas) and CNG (compressed natural gas), the main sources of energy for domestic fuels and the automobile industry, are obtained from petroleum. Hydrocarbons are classified as open chain saturated (alkanes) and unsaturated (alkenes and alkynes), cyclic (alicyclic) and aromatic, according to their structure. The important reactions of alkanes are free radical substitution, combustion, oxidation and aromatization. Alkenes and alkynes undergo addition reactions, which are mainly electrophilic additions. Aromatic hydrocarbons, despite having unsaturation, undergo mainly electrophilic substitution reactions. These undergo addition reactions only under special conditions. Alkanes show conformational isomerism due to free rotation along the C–C sigma bonds. Out of staggered and the eclipsed conformations of ethane, staggered conformation is more stable as hydrogen atoms are farthest apart. Alkenes exhibit geometrical (cis-trans) isomerism due to restricted rotation around the carbon–carbon double bond. Benzene and benzenoid compounds show aromatic character. Aromaticity, the property of being aromatic is possessed by compounds having specific electronic structure characterised by Hückel (4n+2)π electron rule. The nature of groups or substituents attached to benzene ring is responsible for activation or deactivation of the benzene ring towards further electrophilic substitution and also for orientation of the incoming group. Some of the polynuclear hydrocarbons having fused benzene ring system have carcinogenic property. EXERCISES 9.1 How do you account for the formation of ethane during chlorination of methane ? 9.2 Write IUPAC names of the following compounds : (a) CH3CH=C(CH3)2 (b) CH2=CH-C≡C-CH3 (c) (d) –CH2–CH2–CH=CH2 (f) CH3(CH2)4 CH (CH2)3 CH3 (e) CH2 –CH (CH3)2 (g) CH3 – CH = CH – CH2 – CH = CH – CH – CH2 – CH = CH2 | C2H5 9.3 For the following compounds, write structural formulas and IUPAC names for all possible isomers having the number of double or triple bond as indicated : (a) C4H8 (one double bond) (b) C5H8 (one triple bond) 9.4 Write IUPAC names of the products obtained by the ozonolysis of the following compounds : (i) Pent-2-ene (ii) 3,4-Dimethylhept-3-ene (iii) 2-Ethylbut-1-ene (iv) 1-Phenylbut-1-ene Reprint 2025-26 Hydrocarbons 327 9.5 An alkene ‘A’ on ozonolysis gives a mixture of ethanal and pentan-3-one. Write structure and IUPAC name of ‘A’. 9.6 An alkene ‘A’ contains three C – C, eight C – H σ bonds and one C – C π bond. ‘A’ on ozonolysis gives two moles of an aldehyde of molar mass 44 u. Write IUPAC name of ‘A’. 9.7 Propanal and pentan-3-one are the ozonolysis products of an alkene? What is the structural formula of the alkene? 9.8 Write chemical equations for combustion reaction of the following hydrocarbons: (i) Butane (ii) Pentene (iii) Hexyne (iv) Toluene 9.9 Draw the cis and trans structures of hex-2-ene. Which isomer will have higher b.p. and why? 9.10 Why is benzene extra ordinarily stable though it contains three double bonds? 9.11 What are the necessary conditions for any system to be aromatic? 9.12 Explain why the following systems are not aromatic? (i) (ii) (iii) 9.13 How will you convert benzene into (i) p-nitrobromobenzene (ii) m- nitrochlorobenzene (iii) p - nitrotoluene (iv) acetophenone? 9.14 In the alkane H3C – CH2 – C(CH3)2 – CH2 – CH(CH3)2, identify 1°,2°,3° carbon atoms and give the number of H atoms bonded to each one of these. 9.15 What effect does branching of an alkane chain has on its boiling point? 9.16 Addition of HBr to propene yields 2-bromopropane, while in the presence of benzoyl peroxide, the same reaction yields 1-bromopropane. Explain and give mechanism. 9.17 Write down the products of ozonolysis of 1,2-dimethylbenzene (o-xylene). How does the result support Kekulé structure for benzene? 9.18 Arrange benzene, n-hexane and ethyne in decreasing order of acidic behaviour. Also give reason for this behaviour. 9.19 Why does benzene undergo electrophilic substitution reactions easily and nucleophilic substitutions with difficulty? 9.20 How would you convert the following compounds into benzene? (i) Ethyne (ii) Ethene (iii) Hexane 9.21 Write structures of all the alkenes which on hydrogenation give 2-methylbutane. 9.22 Arrange the following set of compounds in order of their decreasing relative reactivity with an electrophile, E+ (a) Chlorobenzene, 2,4-dinitrochlorobenzene, p-nitrochlorobenzene (b) Toluene, p-H3C – C6H4 – NO2, p-O2N – C6H4 – NO2. 9.23 Out of benzene, m–dinitrobenzene and toluene which will undergo nitration most easily and why? 9.24 Suggest the name of a Lewis acid other than anhydrous aluminium chloride which can be used during ethylation of benzene. 9.25 Why is Wurtz reaction not preferred for the preparation of alkanes containing odd number of carbon atoms? Illustrate your answer by taking one example. Reprint 2025-26

9.4 Alkynes 1 2 3 4 5 Like alkenes, alkynes are also unsaturated II. H3C–C≡ C– CH2– CH3 Pent–2-yne hydrocarbons. They contain at least one triple 4 3 2 1 bond between two carbon atoms. The number III. H3C–CH–C≡ CH 3-Methyl but–1-yne |of hydrogen atoms is still less in alkynes as CH3compared to alkenes or alkanes. Their general Structures I and II are position isomers formula is CnH2n–2. and structures I and III or II and III are chain The first stable member of alkyne series isomers. is ethyne which is popularly known as acetylene. Acetylene is used for arc welding Problem 9.13 purposes in the form of oxyacetylene flame Write structures of different isomers obtained by mixing acetylene with oxygen corresponding to the 5 th member of gas. Alkynes are starting materials for a large alkyne series. Also write IUPAC names of number of organic compounds. Hence, it all the isomers. What type of isomerism is interesting to study this class of organic is exhibited by different pairs of isomers? compounds. Solution 9.4.1 Nomenclature and Isomerism th 5 member of alkyne has the molecular In common system, alkynes are named as formula C6H10. The possible isomers are: derivatives of acetylene. In IUPAC system, they Table 9.2 Common and IUPAC Names of Alkynes (CnH2n–2) Value of n Formula Structure Common name IUPAC name 2 C2H2 H-C≡CH Acetylene Ethyne 3 C3H4 CH3-C≡CH Methylacetylene Propyne 4 C4H6 CH3CH2-C≡CH Ethylacetylene But-1-yne 4 C4H6 CH3-C≡C-CH3 Dimethylacetylene But-2-yne Reprint 2025-26 Hydrocarbons 315 (a) HC ≡ C – CH2 – CH2 – CH2 – CH3 Hex-1-yne (b) CH3 – C ≡ C – CH2 – CH2 – CH3 Hex-2-yne (c) CH3 – CH2 – C ≡ C – CH2– CH3 Hex-3-yne 3-Methylpent-1-yne 4-Methylpent-1-yne 4-Methylpent-2-yne Fig. 9.6 Orbital picture of ethyne showing (a) sigma overlaps (b) pi overlaps. orbitals of the other carbon atom, which undergo lateral or sideways overlapping to 3,3-Dimethylbut-1-yne form two pi (π) bonds between two carbon atoms. Thus ethyne molecule consists of one Position and chain isomerism shown by C–C σ bond, two C–H σ bonds and two C–C different pairs. π bonds. The strength of C≡C bond (bond enthalpy 823 kJ mol -1) is more than those 9.4.2 Structure of Triple Bond of C=C bond (bond enthalpy 681 kJ mol –1) Ethyne is the simplest molecule of alkyne and C–C bond (bond enthalpy 348 kJ mol–1). series. Structure of ethyne is shown in The C≡C bond length is shorter (120 pm) Fig. 9.6. than those of C=C (133 pm) and C–C (154 pm). Electron cloud between two carbon Each carbon atom of ethyne has two sp atoms is cylindrically symmetrical about thehybridised orbitals. Carbon-carbon sigma (σ) internuclear axis. Thus, ethyne is a linear bond is obtained by the head-on overlapping molecule. of the two sp hybridised orbitals of the two carbon atoms. The remaining sp hybridised 9.4.3 Preparation orbital of each carbon atom undergoes 1. From calcium carbide: On industrial overlapping along the internuclear axis with scale, ethyne is prepared by treating the 1s orbital of each of the two hydrogen calcium carbide with water. Calcium atoms forming two C-H sigma bonds. carbide is prepared by heating quick lime H-C-C bond angle is of 180°. Each carbon with coke. Quick lime can be obtained byhas two unhybridised p orbitals which are heating limestone as shown in the followingperpendicular to each other as well as to the reactions:plane of the C-C sigma bond. The 2p orbitals of one carbon atom are parallel to the 2p CaCO3 ∆ CaO + O2 (9.55) Reprint 2025-26 316 chemistry CaO + 3C CaC2 + CO (9.56) the sp hybridised carbon2 atoms whereas they are attached to sp hybridised carbon Calcium 3 atoms in ethene and sp hybridised carbons carbide in ethane. Due to the maximum percentage of CaC2 + 2H2O Ca(OH)2 + C2H2 (9.57) s character (50%), the sp hybridised orbitals of carbon atoms in ethyne molecules have2. From vicinal dihalides : Vicinal dihalides highest electronegativity; hence, these attract on treatment with alcoholic potassium the shared electron pair of the C-H bond of hydroxide undergo dehydrohalogenation. ethyne to a greater extent than that of the One molecule of hydrogen halide is 2 sp hybridised orbitals of carbon in ethene eliminated to form alkenyl halide which 3 and the sp hybridised orbital of carbon in on treatment with sodamide gives alkyne. ethane. Thus in ethyne, hydrogen atoms can be liberated as protons more easily as compared to ethene and ethane. Hence, hydrogen atoms of ethyne attached to triply bonded carbon atom are acidic in nature. You may note that the hydrogen atoms attached to the triply bonded carbons are acidic but not all the hydrogen atoms of alkynes. HC ≡ CH + Na → HC ≡ C–Na++ 1/2 H2 9.4.4 Properties Monosodium Physical properties ethynide Physical properties of alkynes follow the same (9.59) trend of alkenes and alkanes. First three HC ≡ C– Na + Na → Na+ C–Na+ ≡ C–Na++ 1/2 H2members are gases, the next eight are liquids and the higher ones are solids. All alkynes Disodium ethynide are colourless. Ethyene has characteristic (9.60)odour. Other members are odourless. Alkynes are weakly polar in nature. They are lighter CH3 – C ≡ C – H + Na+ NH–2 than water and immiscible with water but ↓ soluble in organic solvents like ethers, carbon CH3 – C ≡ C– Na+ + NH3 tetrachloride and benzene. Their melting Sodium propynide (9.61) point, boiling point and density increase with These reactions are not shown by alkenesincrease in molar mass. and alkanes, hence used for distinction Chemical properties between alkynes, alkenes and alkanes. What Alkynes show acidic nature, addition reactions about the above reactions with but-1-yne and and polymerisation reactions as follows : but-2-yne ? Alkanes, alkenes and alkynes A. Acidic character of alkyne: Sodium follow the following trend in their acidic metal and sodamide (NaNH2) are strong behaviour : bases. They react with ethyne to form sodium i) CH ≡ CH > H2C – CH2 > CH3 –CH3acetylide with the liberation of dihydrogen gas. These reactions have not been observed ii) HC ≡ CH > CH3 –C≡ CH >> CH3 –C≡C–CH3in case of ethene and ethane thus indicating that ethyne is acidic in nature in comparison B. Addition reactions: Alkynes contain a to ethene and ethane. Why is it so ? Has triple bond, so they add up, two molecules of it something to do with their structures dihydrogen, halogen, hydrogen halides etc. and the hybridisation ? You have read that Formation of the addition product takes place hydrogen atoms in ethyne are attached to according to the following steps. Reprint 2025-26 Hydrocarbons 317 The addition product formed depends upon stability of vinylic cation. Addition in unsymmetrical alkynes takes place according to Markovnikov rule. Majority of the reactions of alkynes are the examples of electrophilic addition reactions. A few addition reactions (9.66)are given below: (i) Addition of dihydrogen (iv) Addition of water Pt/Pd/Ni H2 Like alkanes and alkenes, alkynes are alsoHC≡CH+H2 [H2C = CH2] CH3–CH3 immiscible and do not react with water. (9.62) However, one molecule of water adds to alkynes on warming with mercuric sulphate CH3–C≡CH + H2 Pt/Pd/Ni [CH3–CH=CH2] and dilute sulphuric acid at 333 K to form Propyne Propene carbonyl compounds. ↓H2 CH3–CH2–CH3 Propane (9.63) (ii) Addition of halogens (9.67) (9.64) Reddish orange colour of the solution of bromine in carbon tetrachloride is decolourised. This is used as a test for unsaturation. (iii) Addition of hydrogen halides (9.68) Two molecules of hydrogen halides (HCl, HBr, (v) Polymerisation HI) add to alkynes to form gem dihalides (in (a) Linear polymerisation: Under suitable which two halogens are attached to the same conditions, linear polymerisation of ethyne carbon atom) takes place to produce polyacetylene or H–C≡C–H+H–Br [CH2 = CH–Br]→ CHBr2 polyethyne which is a high molecular Bromoethene weight polyene containing repeating units of CH3 (CH = CH – CH = CH ) and can be represented 1,1-Dibromoethane as —(CH = CH – CH = CH)n— Under special (9.65) conditions, this polymer conducts electricity. Reprint 2025-26 318 chemistry Thin film of polyacetylene can be used as but in a majority of reactions of aromatic electrodes in batteries. These films are good compounds, the unsaturation of benzene ring conductors, lighter and cheaper than the is retained. However, there are examples of metal conductors. aromatic hydrocarbons which do not contain a (b) Cyclic polymerisation: Ethyne on benzene ring but instead contain other highly unsaturated ring. Aromatic compoundspassing through red hot iron tube at 873K containing benzene ring are known asundergoes cyclic polymerization. Three benzenoids and those not containing amolecules polymerise to form benzene, which benzene ring are known as non-benzenoids.is the starting molecule for the preparation of Some examples of arenes are givenderivatives of benzene, dyes, drugs and large below:number of other organic compounds. This is the best route for entering from aliphatic to aromatic compounds as discussed below: Benzene Toluene Naphthalene (9.69) Biphenyl Problem 9.14 How will you convert ethanoic acid into 9.5.1 Nomenclature and Isomerism benzene? The nomenclature and isomerism of aromatic Solution hydrocarbons has already been discussed in Unit 8. All six hydrogen atoms in benzene are equivalent; so it forms one and only one type of monosubstituted product. When two hydrogen atoms in benzene are replaced by two similar or different monovalent atoms or groups, three different position isomers are possible. The 1, 2 or 1, 6 is known as the ortho (o–), the 1, 3 or 1, 5 as meta (m–) and the 1, 4 as para (p–) disubstituted compounds. A few examples of derivatives of benzene are given below:

1.5 A solution of glucose in water is labelled as 10% w/w, what would be the molality and mole fraction of each component in the solution? If the density of solution is 1.2 g mL–1, then what shall be the molarity of the solution? 1.6 How many mL of 0.1 M HCl are required to react completely with 1 g mixture of Na2CO3 and NaHCO3 containing equimolar amounts of both? 1.7 A solution is obtained by mixing 300 g of 25% solution and 400 g of 40% solution by mass. Calculate the mass percentage of the resulting solution. 1.8 An antifreeze solution is prepared from 222.6 g of ethylene glycol (C2H6O2) and 200 g of water. Calculate the molality of the solution. If the density of the solution is 1.072 g mL–1, then what shall be the molarity of the solution? 1.9 A sample of drinking water was found to be severely contaminated with chloroform (CHCl3) supposed to be a carcinogen. The level of contamination was 15 ppm (by mass): (i) express this in percent by mass (ii) determine the molality of chloroform in the water sample. 1.10 What role does the molecular interaction play in a solution of alcohol and water? 1.11 Why do gases always tend to be less soluble in liquids as the temperature is raised? 1.12 State Henry’s law and mention some important applications. 1.13 The partial pressure of ethane over a solution containing 6.56 × 10–3 g of ethane is 1 bar. If the solution contains 5.00 × 10–2 g of ethane, then what shall be the partial pressure of the gas? 1.14 What is meant by positive and negative deviations from Raoult's law and how is the sign of DmixH related to positive and negative deviations from Raoult's law? 1.15 An aqueous solution of 2% non-volatile solute exerts a pressure of 1.004 bar at the normal boiling point of the solvent. What is the molar mass of the solute? 1.16 Heptane and octane form an ideal solution. At 373 K, the vapour pressures of the two liquid components are 105.2 kPa and 46.8 kPa respectively. What will be the vapour pressure of a mixture of 26.0 g of heptane and 35 g of octane? 1.17 The vapour pressure of water is 12.3 kPa at 300 K. Calculate vapour pressure of 1 molal solution of a non-volatile solute in it. 1.18 Calculate the mass of a non-volatile solute (molar mass 40 g mol–1) which should be dissolved in 114 g octane to reduce its vapour pressure to 80%. 1.19 A solution containing 30 g of non-volatile solute exactly in 90 g of water has a vapour pressure of 2.8 kPa at 298 K. Further, 18 g of water is then added to the solution and the new vapour pressure becomes 2.9 kPa at 298 K. Calculate: (i) molar mass of the solute (ii) vapour pressure of water at 298 K. 1.20 A 5% solution (by mass) of cane sugar in water has freezing point of 271K. Calculate the freezing point of 5% glucose in water if freezing point of pure water is 273.15 K. 1.21 Two elements A and B form compounds having formula AB2 and AB4. When dissolved in 20 g of benzene (C6H6), 1 g of AB2 lowers the freezing point by 2.3 K whereas 1.0 g of AB4 lowers it by 1.3 K. The molar depression constant for benzene is 5.1 K kg mol–1. Calculate atomic masses of A and B. Chemistry 28 Reprint 2025-26 1.22 At 300 K, 36 g of glucose present in a litre of its solution has an osmotic pressure of 4.98 bar. If the osmotic pressure of the solution is 1.52 bars at the same temperature, what would be its concentration?