List of top Chemistry Questions asked in CBSE Class XI

Calcium carbonate reacts with aqueous \(HCl\) to give \(CaCl_2 \) and \(CO_2\) according to the reaction, \(CaCO_3 (s) + 2 HCl (aq) → CaCl_2 (aq) + CO_2(g) + H_2O(l)\). What mass of \(CaCO_3\) is required to react completely with \(25 \ mL\) of \(0.75 \ M\) \(HCl\)?

A sample of pure PCl₅ was introduced into an evacuated vessel at 473 K. After equilibrium was attained, concentration of PCl₅ was found to be \(0.5×10^{–1}\ mol L^{–1}\). If value of K_c is \(8.3×10^{–3}\), what are the concentrations of PCl₃ and Cl₂ at equilibrium?
\(PCl_5 (g) ⇋ PCl_3 (g) + Cl_2(g)\)

Nitric oxide reacts with Br₂ and gives nitrosyl bromide as per reaction given below:
\(2NO (g) + Br_2 (g) ⇋ 2NOBr (g)\)
When 0.087 mol of NO and 0.0437 mol of Br₂ are mixed in a closed container at constant temperature, 0.0518 mol of NOBr is obtained at equilibrium. Calculate equilibrium amount of NO and Br₂ .

For an isolated system, ∆U=0, what will be ∆S ?

Classify the following reactions in one of the reaction type studied in this unit
a) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}+\mathrm{HS}^{-} \rightarrow \mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{SH}+\mathrm{Br}^{-}\)
b) \(\left(\mathrm{CH}_{3}\right)_{2} \mathrm{C}=\mathrm{CH}_{2}+\mathrm{HCl} \rightarrow\left(\mathrm{CH}_{3}\right)_{2} \mathrm{ClC}-\mathrm{CH}_{3}\)
c) \(\mathrm{CH}_{3} \mathrm{CH}_{2} \mathrm{Br}+\mathrm{HO}^{-} \rightarrow \mathrm{CH}_{2}=\mathrm{CH}_{2}+\mathrm{H}_{2} \mathrm{O}+\mathrm{Br}^{-}\)
d) \(\left(\mathrm{CH}_{3}\right)_{3} \mathrm{C}-\mathrm{CH}_{2} \mathrm{OH}+\mathrm{HBr} \rightarrow\left(\mathrm{CH}_{3}\right)_{2} \mathrm{CBrCH}_{2} \mathrm{CH}_{3}+\mathrm{H}_{2} \mathrm{O}\)

Chlorine is prepared in the laboratory by treating manganese dioxide \((MnO_2)\) with aqueous hydrochloric acid according to the reaction
\(4 HCl (aq) + MnO_2(s) → 2H_2O (l) + MnCl_2(aq) + Cl_2 (g)\).
How many grams of HCl react with 5.0 g of manganese dioxide?

Which one of the following statements is incorrect in relation to ionization enthalpy?

Would you expect the first ionization enthalpies for two isotopes of the same element to be the same or different? Justify your answer.

(i) Write the electronic configurations of the following ions: (a) H^– (b) Na⁺ (c) O₂^– (d) F^– 
(ii) What are the atomic numbers of elements whose outermost electrons are represented by (a) 3s¹ (b) 2p³ and (c) 3p⁵? 
(iii) Which atoms are indicated by the following configurations? (a) [He] 2s¹ (b) [Ne] 3s² 3p³ (c) [Ar] 4s² 3d¹.

In the modern periodic table, the period indicates the value of:

The equilibrium constant for the following reaction is 1.6 ×10⁵ at 1024 K.
\(H_2(g) + Br_2(g) ⇋ 2HBr(g)\)
Find the equilibrium pressure of all gases if 10.0 bar of HBr is introduced into a sealed container at 1024 K.

What are the oxidation numbers of the underlined elements in each of the following and how do you rationalise your results? 
(a) KI₃ (b) H₂S₄O₆ (c) Fe₃O₄ (d) CH₃CH₂OH (e) CH₃COOH

Calculate the amount of carbon dioxide that could be produced when 
(i) 1 mole of carbon is burnt in air. 
(ii) 1 mole of carbon is burnt in 16 g of dioxygen. 
(iii) 2 moles of carbon are burnt in 16 g of dioxygen.

The total number of protons in $ 10\,g$ of calcium carbonate is $ (N_{0}=6.023\times 10^{23})$ :

Lifetimes of the molecules in the excited states are often measured by using pulsed radiation source of duration nearly in the nano-second range. If the radiation source has a duration of 2 ns and the number of photons emitted during the pulse source is 2.5 × 10¹⁵, calculate the energy of the source.

H₃PO₃ can be represented by structures 1 and 2 shown below. Can these two structures be taken as the canonical forms of the resonance hybrid representing H₃PO₃ ? If not, give reasons for the same.

Reaction between \(N_2\) and \(O_2^–\) takes place as follows: 
\(2N_2 (g) + O_2 (g) ⇋ 2N_2O (g)\)
If a mixture of \(0.482 \ mol\) \(N_2\) and \(0.933\  mol\) of \(O_2\) is placed in a \(10\  L\) reaction vessel and allowed to form \(N_2O\) at a temperature for which \(K_c = 2.0 × 10^{–37}\), determine the composition of equilibrium mixture.

Explain why the following systems are not aromatic?

Match the following prefixes with their multiples:

S.No	Prefixes	Multiples
(i)	micro	106
(ii)	deca	109
(iii)	mega	10–6
(iv)	giga	10–15
(v)	femto	10

The ionization constant of benzoic acid is 6.46 × 10^–5 and K_sp for silver benzoate is 2.5 × 10^–13. How many times is silver benzoate more soluble in a buffer of pH 3.19 compared to its solubility in pure water?

Using the standard electrode potentials given below ,predict if the reaction between the following is feasible:
(a) Fe³⁺(aq) and I^- (aq) 
(b) Ag⁺ (aq) and Cu(s)
(c) Fe³⁺ (aq) and Cu(s) 
(d) Ag(s) and Fe³⁺(aq)
(e) Br₂(aq) and Fe²⁺(aq)

What sorts of information can you draw from the following reaction?
(CN)₂( g)+2OH⁻(aq)\(\rightarrow\)CN⁻(2aq)+CNO⁻(aq)+H₂O(l)

The solubility product constant of Ag₂CrO₄ and AgBr are 1.1×10^–12 and 5.0×10^–13 respectively. Calculate the ratio of the molarities of their saturated solutions.

Balance the following redox reactions by ion-electron method: 
(a)MnO₄^- _(aq) + I ^- _(aq) → MnO_2(s) + I₂(s) (in basic medium)
(b) MnO₄^- _(aq) + SO_2(g) → Mn₂⁺_(aq) +HSO₄^- _(aq) (in acidic solution)
(c) H₂O_2(aq)+Fe₂⁺_(aq) → Fe₃⁺ _(aq) + H₂O (l) (in acidic solution)
(d) Cr₂O₇^2-+ SO_2(g) → Cr³⁺ _(aq) +SO₄^2- _(aq) (in acidic solution)

Which of the following carbocation is most stable?