List of top Chemistry Questions asked in CBSE Class XI

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

Convert the following into basic units:

1. 28.7 pm
2. 15.15 pm
3. 25365 mg

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

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)

How are 0.50 mol Na₂CO₃ and 0.50 M Na₂CO₃ different?

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⁺

Write the resonance structures for SO₃ , NO₂ and NO₃^-

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) + H₂SO_4(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: N^3- , O^2-, 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.
\(AlCl_3+Cl^- \rightarrow AlCl_4^-\)

Does the number of moles of reaction products increase, decrease or remain same when each of the following equilibria is subjected to a decrease in pressure by increasing the volume?

1. \(PCl_5 (g) ⇋ PCl_3 (g) + Cl_2 (g)\)
2. \(CaO (s) + CO_2 (g) ⇋ CaCO_3 (s)\)
3. \(3Fe (s) + 4H_2O (g) ⇋ Fe_3O_4 (s) + 4H_2 (g)\)

The first ionization constant of H₂S is 9.1 × 10^–8. Calculate the concentration of HS^– ion in its 0.1M solution. How will this concentration be affected if the solution is 0.1M in HCl also? If the second dissociation constant of H₂S is 1.2 × 10^–13, calculate the concentration of S₂^– under both conditions.

At 700 K, equilibrium constant for the reaction:
\(H_2 (g) + I_2 (g) ⇋ 2HI (g)\)
is 54.8. If 0.5 mol L^–1 of HI(g) is present at equilibrium at 700 K, what are the concentration of H₂(g) and I₂(g) assuming that we initially started with HI(g) and allowed it to reach equilibrium at 700 K?

The ejection of the photoelectron from the silver metal in the photoelectric effect experiment can be stopped by applying the voltage of 0.35 V when the radiation 256.7 nm is used. Calculate the work function for silver metal.

The first ionization enthalpy values (in kJmol^-1 ) of group 13 elements are :

B	Al	Ga	In	Tl
801	577	579	558	589

How would you explain this deviation from the general trend?

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

Determine the empirical formula of an oxide of iron, which has 69.9% iron and 30.1% dioxygen by mass.