The 15 one-mark questions from the March 2024 12th Standard Chemistry public exam, in paper order, with the correct option marked and a short explanation.
Q1
Assertion: Monoclinic sulphur is an example of monoclinic crystal system.
Reason: For a monoclinic system, \(a \neq b \neq c\) and \(\alpha = \gamma = 90^\circ, \beta \neq 90^\circ\).
- A. Assertion is true but Reason is false.
- B. Both Assertion and Reason are true and Reason is the correct explanation of Assertion.Correct
- C. Both Assertion and Reason are false.
- D. Both Assertion and Reason are true, but Reason is not the correct explanation of Assertion.
Explanation. Monoclinic sulphur crystallises with unequal axes a ≠ b ≠ c and angles α = γ = 90°, β ≠ 90°, which are exactly the parameters of the monoclinic system. So both statements are true and the reason explains why it is classed as monoclinic.
Q2
IUPAC name of the complex \(K_3[Al(C_2O_4)_3]\) is :
- A. Potassium trisoxalato aluminate (III)
- B. Potassium trioxalato aluminium (III)
- C. Potassium trioxalato aluminate (III)Correct
- D. Potassium trioxalato aluminate (II)
Explanation. The complex ion is anionic, so the metal name ends in -ate (aluminate). Three oxalate ligands give 'trioxalato', and x + 3(−2) = −3 gives Al in the +3 state: potassium trioxalato aluminate (III).
Q3
Which of the following is the strongest acid among all ?
- A. HBr
- B. HICorrect
- C. HCl
- D. HF
Explanation. Down group 17 the H–X bond becomes longer and weaker, so HI releases H⁺ most easily. HI is the strongest acid among the hydrogen halides.
Q4
Which of the following is not \(sp^2\) hybridised ?
- A. Fullerene
- B. Graphite
- C. Dry iceCorrect
- D. Graphene
Explanation. Fullerene, graphite and graphene have sp² hybridised carbon atoms. Dry ice is solid CO₂, where carbon (O=C=O) is sp hybridised.
Q5
The pyrimidine bases present in RNA are :
- A. Cytosine and Thymine
- B. Cytosine and Adenine
- C. Cytosine and UracilCorrect
- D. Cytosine and Guanine
Explanation. RNA contains the purines adenine and guanine and the pyrimidines cytosine and uracil. Thymine occurs in DNA, not RNA.
Q6
Aspirin is :
- A. chlorobenzoic acid
- B. acetyl salicylic acidCorrect
- C. anthranilic acid
- D. benzoyl salicylic acid
Explanation. Aspirin is obtained by acetylating the –OH group of salicylic acid with acetic anhydride, so it is acetyl salicylic acid (2-acetoxybenzoic acid).
Q7
In acid medium, potassium permanganate oxidizes oxalic acid to :
- A. acetate
- B. oxalate
- C. acetic acid
- D. carbon dioxideCorrect
Explanation. In acid medium \(MnO_4^-\) is reduced to \(Mn^{2+}\) and oxalic acid is oxidised: \(5C_2O_4^{2-} + 2MnO_4^- + 16H^+ \rightarrow 2Mn^{2+} + 10CO_2 + 8H_2O\). The product is carbon dioxide.
Q8
When aniline reacts with acetic anhydride, the product formed is :
- A. p-aminoacetophenone
- B. o-aminoacetophenone
- C. acetanilideCorrect
- D. m-aminoacetophenone
Explanation. Aniline undergoes N-acetylation: \(C_6H_5NH_2 + (CH_3CO)_2O \rightarrow C_6H_5NHCOCH_3 + CH_3COOH\). The product is acetanilide.
Q9
Among the following which will not be hydrolysed ?
- A. Sodium ChlorideCorrect
- B. Sodium Formate
- C. Ammonium Formate
- D. Ammonium Nitrate
Explanation. NaCl is a salt of a strong acid (HCl) and a strong base (NaOH), so neither ion reacts with water. The other salts contain a weak acid anion or a weak base cation and are hydrolysed.
Q10
Among the following cells primary cells are :
(i) Leclanche cell
(ii) Nickel-Cadmium cell
(iii) Lead Storage Battery
(iv) Mercury cell
- A. (iii) and (iv)
- B. (i) and (iv)Correct
- C. (ii) and (iii)
- D. (i) and (iii)
Explanation. Primary cells cannot be recharged: the Leclanche cell and the mercury cell. The Ni-Cd cell and the lead storage battery are rechargeable secondary cells.
Q11
The formation of cyanohydrin from acetone is an example of :
- A. electrophilic addition
- B. nucleophilic substitution
- C. nucleophilic additionCorrect
- D. electrophilic substitution
Explanation. The nucleophile \(CN^-\) attacks the electrophilic carbonyl carbon of acetone and the product adds \(H^+\) on oxygen, giving acetone cyanohydrin. This is nucleophilic addition.
Q12
The rate constant of a reaction is \(5.8 \times 10^{-2}\ s^{-1}\). The order of the reaction is :
- A. Second order
- B. First orderCorrect
- C. Third order
- D. Zero order
Explanation. The unit of rate constant is \((mol\ L^{-1})^{1-n}\ s^{-1}\). A unit of \(s^{-1}\) means 1 − n = 0, so n = 1: first order.
Q13
In the electrolytic refining of copper, which one of the following is used as anode ?
- A. Carbon rod
- B. Pure copper
- C. Platinum electrode
- D. Impure copperCorrect
Explanation. In electrolytic refining the impure metal is made the anode and a thin strip of pure metal the cathode. Copper dissolves from the impure anode and deposits as pure copper on the cathode.
Q14
Activity of iron catalyst is increased by the ________ compound.
- A. \(CH_3COOH\)
- B. \(H_2S\)
- C. \(Al_2O_3\)Correct
- D. \(As_2O_3\)
Explanation. In the Haber process, \(Al_2O_3\) acts as a promoter and increases the activity of the iron catalyst. \(H_2S\) and \(As_2O_3\) are catalyst poisons.
Q15
The oxidising agent used to stop the oxidation of primary alcohol at the aldehyde stage is :
- A. \(Na_2Cr_2O_7\)
- B. \(KMnO_4\)
- C. \(K_2Cr_2O_7\)
- D. PCCCorrect
Explanation. Pyridinium chlorochromate (PCC) is a mild oxidising agent that converts a primary alcohol only to the aldehyde. Acidified dichromate or KMnO₄ oxidise it further to the carboxylic acid.
