NEB Grade 12 Chemistry MCQs with Answer Exam 2081
Group 'A'
Rewrite the correct option of each question in your answer sheet.
1. A chemical reaction occurs as follows:
NaOH + H3PO4 → NaH2PO4 + H2O
What is the equivalent weight of H3PO4?
A. 25
B. 49
C. 58
D. 98
Answer
In the given reaction, only one proton (H+) from H3PO4 is replaced by a sodium ion (Na+). So the basicity of H3PO4 in this reaction is one.
To calculate the equivalent weight of H3PO4, we use the formula:
Equivalent Weight = Molar Mass / Basicity
Molar mass of H3PO4 = 98 g/mol.
So, the Equivalent weight of H3PO4 = 98/1 = 98 g/equivalent
2. If 0.01M solution of acetic acid is 0.01% ionized, what will be the dissociation constant of acetic acid?
A. 1×10-3
B. 1×10-4
C. 1×10-8
D. 1×10-10
Answer
Given:
Initial concentration of acetic acid (C) = 0.01 M
Percentage of ionization = 0.01%
So, the degree of ionization (α) from the percentage of ionization:
α = percentage of ionization/100% = 0.01/100 = 0.0001 = 10−4
Acetic acid (CH3COOH) is a weak acid that dissociates in water according to the following equilibrium:
CH3COOH ⇌ H+ + CH3COO−
Dissociation constant (Ka) = Cα2/(1 − α)
Since, the degree of ionization (α = 0.0001) is very small compared to 1.
So, we can approximate (1−α) ≈ 1.
Therefore, Ka ≈ Cα2
putting the value of C and α, we get-
10−2 × (10−4)2 = 10−10
So, the dissociation constant of acetic acid is approximately, 1 × 10−10
3. If the rate of reaction is equal to the rate constant, what will be the order of reaction?
A. Zero
B. First
C. Second
D. Third
Answer
Let us consider a general equation
aA + bB → Product
So, the rate law (k) = [A]x[B]y
where, x is the order of the reaction with respect to reactant A and y is the order of the reaction with respect to reactant B.
and the overall order of the reaction (n) = x + y.
We are given that the rate of reaction is equal to the rate constant, that means
Rate = k
Comparing this with the general rate law, we have:
k = k [A]x[B]y
Now, the rate of reaction is equal to the rate constant only when the [A] and [B] becomes one and this can only happen if the orders of the reaction with respect to all reactants are zero.
[A]o = 1
[B]o = 1
Therefore, x = 0 and y = 0.
And the overall order of the reaction (n) = x + y = 0 + 0 = 0
Thus, if the rate of reaction is equal to the rate constant, the order of the reaction is zero.
4. Standard electrode potentials of four metals: P, Q, R and S are +0.34V, -0.25V, -2.93V and +0.85V respectively. Which is the correct arrangement in the order of decreasing reactivity?
A. S > P > Q > R
B. Q > R > P > S
C. R > Q > P > S
D. R > S > Q > P
Answer
We know that the reactivity of a metal is inversely proportional to its standard reduction potential that means metals with a more negative standard reduction potential has a greater tendency to lose electrons and undergo oxidation, making it more reactive. Conversely, a metal with a more positive standard reduction potential has a greater tendency to gain electrons and undergo reduction, making it less reactive.
To arrange them in decreasing order of reactivity, we need to arrange them in increasing order of their standard reduction potentials and then reverse the order.
Increasing order of standard reduction potentials:
R (-2.93V) < Q (-0.25V) < P (+0.34V) < S (+0.85V)
Now, reversing this order to get the decreasing order of reactivity: R > Q > P > S
Therefore, the correct arrangement in the order of decreasing reactivity is R > Q > P > S.
5. What features of transition metals make them suitable to act as catalysts?
A. Large ionic charge
B. Variable oxidation state
C. Highly reactive nature
D. Large surface area for the reactant to be absorbed
Answer
Variable Oxidation State: Transition metals exhibit variable oxidation states due to the incomplete filling of d-orbital. Due to this, they are readily accept and donate electrons. They can form intermediate complexes with reactants, making it easier for the reaction to proceed through a lower activation energy pathway.
Although a large surface area can enhance catalytic activity by providing more surface for reactants, but this is not the defining characteristic of transition metals as catalysts.
6. Which of the following metal ions show a green colour in its salt?
A. Ti3+
B. Cr3+
C. Mn2+
D. Fe2+
Answer
Chromium(III) ions (Cr3+) in aqueous solutions and many of their salts exhibit a green color due to d-d transitions. The specific shade of green can vary depending on the ligands attached to the Cr3+ ion.
Fe2+ also exhibit a green color in many of its salts and aqueous solutions but the intensity of color is weak (pale green) and the exact shade can vary depending on the specific compound and its environment.
7. If the compound Y is heated with acid anhydride in presence of sodium acetate, what will it give?
A. Cinnamic acid
B. Picric acid
C. Benzoic acid
D. Phthalic acid
Answer
8. Which of the following compounds gives positive Tollens test as well as iodoform test?
A. Propanone
B. Ethanol
C. Ethanal
D. Methanal
Answer
Propanone gives a positive iodoform test but a negative Tollens test.
Ethanol gives a positive iodoform test but a negative Tollens test.
Ethanal gives a positive Tollens test and a positive iodoform test.
Methanal gives a positive Tollens test but a negative iodoform test.
Therefore, the compound that gives both positive Tollens test and iodoform test is Ethanal.
9. Which one of the following is most basic in nature?
A. NH3
B. CH3NH2
C. (CH3)2NH
D. C6H5NH2
Answer
Larger the number of lone pair, stronger the base in gaseous medium. Based on the electron-donating effects of alkyl groups and the electron-withdrawing effect due to resonance in aniline, the order of basicity is expected to be:
(CH3)2NH>CH3NH2>NH3>C6H5NH2.
Therefore, dimethylamine (CH3)2NH is the most basic among the given compounds.
10. Which of the following compounds reacts with chlorobenzene to form DDT?
A. Carbonyl chloride
B. Acetone
C. Chloral
D. Chloroform
Answer
DDT (Dichlorodiphenyltrichloroethane) is synthesized by the reaction of chloral with chlorobenzene in the presence of concentrated sulfuric acid as a catalyst.
2 C6H5Cl + CCl3CHO --H2SO4--> (ClC6H4)2CHCCl3 + H2O
11. If you are asked to prepare primary alcohol using Grignard's reagent, what will you start with?
A. Methanal
B. Ethanal
C. Propanone
D. Acetyl chloride
Answer
To prepare a primary alcohol using Grignard's reagent, you should start with Methanal (Formaldehyde).
HCHO + R-Mg-X ---H 3O+---> R-CH2-OH
NEB Grade 12 Chemistry (Technical Stream) Exam Question Paper 2082 (2025)