The 15 one-mark questions from the March 2023 12th Standard Chemistry public exam, in paper order, with the correct option marked and a short explanation.
Q1
Milk of Magnesia is used as ______.
- A. Tranquilizer
- B. Analgesic
- C. AntacidCorrect
- D. Anaesthetic
Explanation. Milk of Magnesia is a suspension of \(Mg(OH)_2\), a weak base that neutralises excess HCl in the stomach, so it is an antacid.
Q2
The addition of a catalyst during a chemical reaction alters, which of the following quantities ?
- A. Entropy
- B. Internal energy
- C. Activation energyCorrect
- D. Enthalpy
Explanation. A catalyst provides an alternative path with lower activation energy; state functions such as entropy, internal energy and enthalpy of the reaction are unchanged.
Q3
The metal extracted by Hall-Heroult process is :
- A. Cu
- B. AlCorrect
- C. Zn
- D. Ni
Explanation. In the Hall-Heroult process, alumina dissolved in molten cryolite is electrolysed to obtain aluminium.
Q4
A magnetic moment of 1.73 BM will be shown by one among the following :
- A. \([CoCl_6]^{4-}\)
- B. \(TiCl_4\)
- C. \([Cu(NH_3)_4]^{2+}\)Correct
- D. \([Ni(CN)_4]^{2-}\)
Explanation. 1.73 BM = \(\sqrt{n(n+2)}\) with n = 1. \(Cu^{2+}\) is \(3d^9\) with one unpaired electron; \(Co^{2+}\) (\(d^7\), high spin) has 3, while \(Ti^{4+}\) (\(d^0\)) and square planar \([Ni(CN)_4]^{2-}\) have none.
Q5
Which one is correctly matched ?
- A. Foam - mist
- B. Emulsion - smoke
- C. Sol - whipped cream
- D. Gel - butterCorrect
Explanation. Butter is a gel (liquid dispersed in solid). Whipped cream is a foam, mist is an aerosol of liquid in gas, and smoke is an aerosol of solid in gas.
Q6
Which one of the following will cause common-ion-effect when added to the following dissociation equilibrium reaction ?
\(CH_3COOH_{(aq)} \rightleftharpoons CH_3COO^-_{(aq)} + H^+_{(aq)}\)
- A. \(CH_3COCl\)
- B. \(AgCl\)
- C. \(CH_3Cl\)
- D. \(HCl\)Correct
Explanation. HCl is a strong acid that supplies \(H^+\) ions, the ion common to the equilibrium, so it suppresses the dissociation of acetic acid.
Q7
Carbon atoms in Fullerene with formula \(C_{60}\) have ______ hybridisation.
- A. \(sp^2\)Correct
- B. \(sp^3\)
- C. \(sp^3d\)
- D. \(sp\)
Explanation. Each carbon in \(C_{60}\) is bonded to three other carbons in five- and six-membered rings, so it is \(sp^2\) hybridised.
Q8
The most common oxidation state of Actinoids is :
- A. +4
- B. +2
- C. +6
- D. +3Correct
Explanation. Like the lanthanoids, the actinoids most commonly show the +3 oxidation state, though they also show other states.
Q9
Assertion : Hex-4-ennitrile on reaction with Di-isobutyl aluminium hydride followed by Hydrolysis gives Hex-4-enal.
Reason : Di-isobutyl aluminium hydride is a selective reducing agent.
- A. Assertion is true but Reason is false.
- B. Both Assertion and Reason are true, but reason is not the correct explanation of assertion.
- C. Both Assertion and Reason are false.
- D. Both Assertion and Reason are true and reason is the correct explanation of assertion.Correct
Explanation. DIBAL-H selectively reduces the nitrile group to an imine, which hydrolyses to the aldehyde, leaving the C=C double bond untouched. Both statements are true and the reason explains the assertion.
Q10
Which of the following is used as the source of gamma rays ?
- A. Xe
- B. Ar
- C. RnCorrect
- D. Kr
Explanation. Radon is radioactive and is used as a source of gamma rays (for example in radiotherapy).
Q11
The emf of Standard Hydrogen Electrode (SHE) is ______.
- A. −1.0
- B. 0Correct
- C. 1.1
- D. +1.0
Explanation. The standard electrode potential of the SHE is taken as 0 V by convention; other electrode potentials are measured against it.
Q12
The crystal with metal excess defect is :
- A. NaClCorrect
- B. AgBr
- C. AgCl
- D. FeO
Explanation. When NaCl is heated in sodium vapour, anion vacancies are occupied by electrons (F-centres), giving a metal excess defect. AgBr shows Frenkel and Schottky defects and FeO shows metal deficiency.
Q13
Which of the following base is not present in DNA ?
- A. UracilCorrect
- B. Adenine
- C. Cytosine
- D. Guanine
Explanation. DNA contains adenine, guanine, cytosine and thymine; uracil occurs only in RNA.
Q14
Which of the following compound is used as an anti-freeze in automobile radiator ?
- A. Methanol
- B. Ethane-1, 2-diolCorrect
- C. Ethanol
- D. Glycerol
Explanation. Ethane-1,2-diol (ethylene glycol) lowers the freezing point of water and is used as anti-freeze in automobile radiators.
Q15
The product formed by the reaction of an Aromatic aldehyde with primary amines is :
- A. Schiff's baseCorrect
- B. Carboxylic acid
- C. Ketone
- D. Aromatic acid
Explanation. An aromatic aldehyde condenses with a primary amine with loss of water to form an imine \(ArCH=NR\), called a Schiff's base (e.g. benzaldehyde + aniline gives benzylidene aniline).
