List of top Chemistry Questions asked in CBSE Class XI

Draw the Lewis structures for the following molecules and ions: \(H_2S\), \(SiCl_4\), \(BeF_2\), \(CO_3^{2-}\) , \(HCOOH\)

The following results are observed when sodium metal is irradiated with different wavelengths. Calculate (a) threshold wavelength and, (b) Planck’s constant
λ (nm)500 450400 
v × 10–5(cm s–1)2.554.355.35
What do you understand by isoelectronic species? Name a species that will be isoelectronic with each of the following atoms or ions. 
  1. F- 
  2. Ar 
  3. Mg2+ 
  4. Rb+
Bromine monochloride, BrCl decomposes into bromine and chlorine and reaches the equilibrium:
\(2BrCl (g) ⇋ Br_2 (g) + Cl_2 (g)\)
for which Kc = 32 at 500 K. If initially pure BrCl is present at a concentration of 3.3 × 10–3 mol L–1, what is its molar concentration in the mixture at equilibrium?
Write the atomic number of the element present in the third period and seventeenth group of the periodic table.
Discuss the shape of the following molecules using the VSEPR model: 
BeCl2, BCl3, SiCl4, AsF5, H2S, PH3
Write the resonance structures for SO3 , NO2 and NO3-
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)
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.
The enthalpy of combustion of methane, graphite and dihydrogen at 298 K are, -890.3 kJ mol-1, -393.5 kJ mol-1, and -285.8 kJ mol-1 respectively. Enthalpy of formation of CH4(g) will be
Convert the following into basic units:
  1. 28.7 pm
  2. 15.15 pm
  3. 25365 mg
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
Using s, p, d notations, describe the orbital with the following quantum numbers.
  1. n=1, l=0
  2. n = 3, l=1
  3. n = 4, l =2
  4. n=4, l=3
A welding fuel gas contains carbon and hydrogen only. Burning a small sample of it in oxygen gives 3.38 g carbon dioxide, 0.690 g of water and no other products. A volume of 10.0 L (measured at STP) of this welding gas is found to weigh 11.6 g. Calculate:
  1. empirical formula, 
  2. molar mass of the gas, and 
  3. molecular formula
How are 0.50 mol Na2CO3 and 0.50 M Na2CO3 different?
Consider the reactions: 
  1. H3PO2(aq) + 4 AgNO3(aq) + 2 H2O(l) \(\rightarrow\) H3PO4(aq) + 4Ag(s) + 4HNO3(aq)
  2. H3PO2(aq) + 2CuSO4(aq) + 2 H2O(l) \(\rightarrow\) H3PO4(aq) + 2Cu(s) + H2SO4(aq) 
  3. C6H5CHO(l) + 2[Ag (NH3)2]+(aq) + 3OH- (aq)  \(\rightarrow\) C6H5COO- (aq) + 2Ag(s) + 4NH3(aq) + 2 H2O(l) 
  4. C6H5CHO(l) + 2Cu2+(aq) + 5OH-(aq) \(\rightarrow\) No change observed. 

What inference do you draw about the behaviour of Ag+ and Cu2+  from these reactions?

Consider the following species: N3- , O2-, F-, Na+, Mg2+ and Al3+ 
  1. What is common in them? 
  2. Arrange them in the order of increasing ionic radii.
Give a brief description of the principles of the following techniques taking an example in each case.(a) Crystallisation (b) Distillation (c) Chromatography.
Draw diagrams showing the formation of a double bond and a triple bond between carbon atoms in \(C_2H_4\) and \(C_2H_2\) molecules.
Justify that the following reactions are redox reactions:
  1. \(CuO(s) + H_2(g) \rightarrow Cu(s) + H_2O(g) \)
  2. \(Fe_2O_3(s) + 3CO(g) \rightarrow 2Fe(s) + 3CO_2(g) \)
  3. \(4BCl_3(g) + 3LiAlH_4(s) \rightarrow 2B_2H_6(g) + 3LiCl(s) + 3 AlCl_3 (s) \)
  4. \(2K(s) + F_2(g) \rightarrow 2K+F^- (s) \)
  5. \(4 NH_3(g) + 5 O_2(g) \rightarrow 4NO(g) + 6H_2O(g)\)
In Ostwald's process for the manufacture of nitric acid, the first step involves the oxidation of ammonia gas by oxygen gas to give nitric oxide gas and steam. What is the maximum weight of nitric oxide that can be obtained starting only with 10.00 g. of ammonia and 20.00 g of oxygen?
Calculate the pH of the following solutions: 
a) 2 g of TlOH dissolved in water to give 2 litre of solution. 
b) 0.3 g of Ca(OH)2 dissolved in water to give 500 mL of solution. 
c) 0.3 g of NaOH dissolved in water to give 200 mL of solution. 
d) 1mL of 13.6 M HCl is diluted with water to give 1 litre of solution
In the organic compound \(CH_2 = CH-CH_2-CH_2-C≡CH\), the pair of hydridised orbitals involved in the formation of \(C_2 – C_3\) bond is
Draw formulas for the first five members of each homologous series beginning with the following compounds.\((a) H–COOH (b) CH_3COCH_3 (c) H–CH=CH_2\)
Describe the change in hybridisation (if any) of the Al atom in the following reaction.
\(AlCl_3+Cl^- \rightarrow AlCl_4^-\)