SOLUTIONS
2025 BOARD EXAM
CBSE CLASS 12-PCB CHEMISTRY Board Paper 2025 — Set 4
2025
CHEMISTRY
CLASS 12-PCB
CBSE EXAMINATION PAPER-2025
CHEMISTRY
(Solved)
General Instructions :
Read the following instructions carefully and follow them :
- This question paper contains 22 questions. All questions are compulsory.
- This question paper is divided into 4 sections.
- Section A – questions number 1 to 3 are case based questions
- Section B – questions number 4 to 16 are multiple choice questions
- Section C – questions number 17 to 19 are very short answer
- Section D – questions number 20 to 22 are short answer
- There is no overall choice given in the question paper. However, an internal choice has been provided in few questions.
- Use of calculator is NOT allowed.
Section A
Carbohydrates are polyhydroxy aldehydes or ketones that represent enormous structural diversity in terms of the arrangement of atoms in space, resulting in hundreds of stereoisomers. Although the chemical properties of most stereoisomers may not be very different, their metabolic rate and utilization in biological systems is significantly different and known to influence the overall carbohydrate metabolism. Structural variants, which arise due to a different arrangement of atoms in three-dimensional space are known as stereoisomers. The number of stereoisomers can be theoretically estimated by using the formula 2n, where ‘n’ is the number of stereocenters or asymmetric (chiral) carbon atoms in a molecule. Out of these stereoisomers, there are some structures, which are mirror images of each other, and they are referred to as enantiomers.
Answer the following questions :
(1) Give chemical reactions to show the presence of an aldehydic groupand straight chain in glucose.
(2) Draw the structure of β-D-Glucopyranose.
(3) Define anomers.
(4) Sucrose is known as invert sugar. Explain.
Werner’s coordination theory in 1893 was the first attempt to explain the bonding in coordination complexes. It must be remembered that this theory was put forward before the electron had been discovered by J.J. Thomson in 1897, and before the electronic theory of valency. Werner did not have any of the modern instrumental techniques and all his studies were made using simple experimental techniques. Werner was able to explain the nature of bonding in complexes and he concluded that in complexes, the metal shows two different sorts of valency : primary and secondary. Primary valences are normally ionisable whereas secondary valences are non ionisable.
Answer the following questions :
(1) One mole of CrCl₃ . 4H₂O precipitates one mole of AgCl whentreated with excess of AgNO₃ solution. Write (i) the structuralformula of the complex, and (ii) the secondary valency of Cr.
(2) What is the difference between a complex and a double salt ?
(3) Arrange the following complexes in the increasing order of conductivity of their solution : [Cr(NH₃)₃Cl₃], [Cr(NH₃)₆]Cl₃, [Cr(NH₃)₅Cl]Cl₂
(4) Write two differences between primary and secondary valences in coordination compounds.
Section B
The role of a catalyst is to change:
Which of the following molecules is chiral in nature?
CH₃CH₂OH can be converted to CH₃CHO by:
The IUPAC name for CH₃-CH₂-N(CH₃)-CH₂-CH₂-CH₃ is:
The correct answer is N-ethyl-N-methylpropan-1-amine. The structure consists of a propanamine base (3 carbon atoms) with an ethyl group (C2H5) and a methyl group (CH3) attached to the nitrogen atom (N). This naming follows the IUPAC conventions where substituents and the longest carbon chain are identified, hence leading to the correct name.
The treatment of ethyl bromide with alcoholic silver nitrite gives:
On heating with an ethanolic solution of silver nitrate, alkyl halides yield nitroalkanes. Some of the alkyl nitrites are also formed because nitrite is an ambidentate ligand. It has a lone pair of electrons on nitrogen as well as oxygen. Thus it can be bonded to an alkyl halide through oxygen to from alkyl nitrite and via Nitrogen to form nitroalkane. Hence, the treatment of ethyl bromide with alcoholic silver nitrite gives Nitroethane.
Which of the following aqueous solutions will have the highest freezing point?
In which of the following groups are both ions coloured in aqueous solution?
I. Cu⁺ II. Ti⁴⁺ III. Co²⁺ IV. Fe²⁺
[Atomic number : Cu = 29, Ti = 22, Co = 27, Fe = 26]
The correct option is 'III and IV' because Co²⁺ (Cobalt ion) and Fe²⁺ (Iron ion) are known to be coloured in aqueous solution. Co²⁺ typically appears blue while Fe²⁺ can appear green or yellow depending on its concentration. Cu⁺ is generally colourless and Ti⁴⁺ is often colourless as well, which eliminates I and II as options.
CH₃CH₂CHO and CH₃CH₂COOH can be distinguished by:
Sodium bicarbonate test: Carboxylic acids (like CH3CH2COOH) react with sodium bicarbonate (NaHCO3) to produce carbon dioxide gas, which causes effervescence. Aldehydes (like CH3CH2CHO) do not react with sodium bicarbonate.
While doing qualitative analysis in chemistry lab, Abhishek added yellow coloured potassium chromate solution into a test tube. He was surprised to see the colour of the solution changing immediately to orange. He realised that the test tube was not clean and contained a few drops of some liquid. Which of the following substances will be the most likely liquid to be present in the test tube before adding potassium chromate solution?
The correct answer is HCl solution. The color change from yellow to orange occurs due to the conversion of chromate ions (CrO4^2-) to dichromate ions (Cr2O7^2-) in acidic conditions. Since hydrochloric acid (HCl) is an acid, its presence in the test tube would facilitate this transformation when potassium chromate is added.
Assertion (A) : For measuring resistance of an ionic solution an AC source is used.
Reason (R) : Concentration of ionic solution will change if DC source is used.
Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). Assertion (A) is correct because an AC source is used to prevent changes in the ionic solution, while Reason (R) is a separate fact that explains a consequence of using DC but does not directly clarify why AC is preferred.
Assertion (A) : Henry’s law constant (KH) decreases with increase in temperature.
Reason (R) : As the temperature increases, solubility of gases in liquids decreases.
Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A). The context reveals that higher KH indicates lower solubility, which means that as temperature increases, KH actually increases, leading to lower solubility of gases. Therefore, even though both statements are true, the reasoning does not correctly explain the assertion.
Assertion (A) : The solubility of aldehydes and ketones in water decreases with increase in size of the alkyl group.
Reason (R) : Aldehydes and ketones have dipole-dipole interaction.
Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of the Assertion (A). The assertion is true as the solubility of aldehydes and ketones decreases with increasing size of the alkyl group due to their hydrophobic nature. However, the reason provided does not directly explain this as it refers to dipole-dipole interactions, which do not account for the significant effect of the hydrophobic alkyl chains on solubility.
Assertion (A) : The boiling points of alkyl halides decrease in the order RI > RBr > RCl > RF.
Reason (R) : The van der Waals forces of attraction decrease in the order RI > RBr > RCl > RF
Both Assertion (A) and Reason (R) are true, and Reason (R) is the correct explanation of the Assertion (A).
The assertion is true as the boiling points of alkyl halides decrease in the order RI > RBr > RCl > RF due to the increasing strength of van der Waals forces with increasing atomic size and mass of the halogen atom. The reason is also true because van der Waals forces are stronger in heavier halogens (like iodine) due to greater polarizability. Since stronger intermolecular forces require more energy to overcome, the boiling point increases. Therefore, the reason correctly explains the assertion.
Section C
Calculate the elevation of boiling point of a solution when 3 g of CaCl₂ (Molar mass = 111 g mol⁻¹) was dissolved in 260 g of water, assuming that CaCl₂ undergoes complete dissociation. (Kb for water = 0·52 K kg mol⁻¹)
Liquids ‘X’ and ‘Y’ form an ideal solution. The vapour pressure of pure ‘X’ and pure ‘Y’ are 120 mm Hg and 160 mm Hg respectively. Calculate the vapour pressure of the solution containing equal moles of ‘X’ and ‘Y’.
Given:
Vapour pressure of pure X, P_X = 120 mm Hg
Vapour pressure of pure Y, P_Y = 160 mm Hg
Number of moles are equal, so mole fraction of X, x_X = 0.5 and mole fraction of Y, x_Y = 0.5
According to Raoult's law, vapor pressure of solution, P = x_X * P_X + x_Y * P_Y = 0.5 * 120 + 0.5 * 160 = 60 + 80 = 140 mm Hg.
Therefore, vapour pressure of the solution is 140 mm Hg.
Explain the mechanism of acid catalysed hydration of ethene.
Section D
Shweta mixed two liquids A and B of 10 mL each. After mixing, the volume of the solution was found to be 20·2 mL.
(i) Why was there a volume change after mixing the liquids ?
(ii) Will there be an increase or decrease of temperature after mixing ?
(iii) Give one example for this type of solution.
(i) How does sprinkling of salt help in clearing the snow covered roads in hilly areas ?
(ii) What happens when red blood cells are kept in 0·5% (mass/vol) NaCl solution ? Justify your answer.
(iii) Write an application of reverse osmosis.
An organic compound ‘A’ (molecular formula C₈H₈O) gives 2,4-DNP test. It does not give Tollen’s test, but gives a yellow precipitate ‘B’ with NaOH and I₂. On drastic oxidation, it gives a carboxylic acid ‘C’ with formula C₇H₆O₂. Identify ‘A’, ‘B’, ‘C’ and write the reactions involved.
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