List of top Chemistry Questions asked in CBSE Class XI

Chlorine is used to purify drinking water. Excess of chlorine is harmful. The excess of chlorine is removed by treating with sulphur dioxide. Present a balanced equation for this redox change taking place in water.
The electron energy in hydrogen atom is given by \(E_n = \frac {(–2.18×10^{–18})}{n^2} \ J\). Calculate the energy required to remove an electron completely from the n = 2 orbit. What is the longest wavelength of light in cm that can be used to cause this transition?
The Mn3+ ion is unstable in solution and undergoes disproportionation to give Mn2+ , MnO2, and H+ ion. Write a balanced ionic equation for the reaction.
What is the energy in joules, required to shift the electron of the hydrogen atom from the first Bohr orbit to the fifth Bohr orbit and what is the wavelength of the light emitted when the electron returns to the ground state ? The ground state electron energy is –2.18×10–11 ergs.
Justify giving reactions that among halogens, fluorine is the best oxidant and among hydrohalic compounds, hydroiodic acid is the best reductant.
The compound AgF2 is an unstable compound. However, if formed, the compound acts as a very strong oxidizing agent. Why?
Calculate the wavenumber for the longest wavelength transition in the Balmer series of atomic hydrogen.
  1. The energy associated with the first orbit in the hydrogen atom is –2.18×10–18 J atom–1. What is the energy associated with the fifth orbit ?
  2. Calculate the radius of Bohr’s fifth orbit for hydrogen atom.
The concentration of sulphide ion in 0.1M HCl solution saturated with hydrogen sulphide is 1.0 × 10–19 M. If 10 mL of this is added to 5 mL of 0.04 M solution of the following: FeSO4, MnCl2, ZnCl2 and CdCl2. in which of these solutions precipitation will take place?
What is the minimum volume of water required to dissolve 1g of calcium sulfate at 298 K? (For calcium sulfate, Ksp is 9.1 × 10–6).
How much energy is required to ionise a H atom if the electron occupies n = 5 orbit? Compare your answer with the ionization enthalpy of H atom (energy required to remove the electron from n = 1 orbit).
Consider the reactions:
\(2S_2O_3^{2-}(aq) + I_2(s) \rightarrow S_4O_6^{2-}(aq) + 2I ^- (aq)\)
\(S_2O_3^{2-}(aq) + 2Br_2(l) + 5 H_2O(l) \rightarrow 2SO_4^{2-}(aq) + 4Br ^- (aq) + 10H ^+ (aq)\) 
Why does the same reductant, thiosulphate react differently with iodine and bromine?
What is the maximum concentration of equimolar solutions of ferrous sulphate and sodium sulphide so that when mixed in equal volumes, there is no precipitation of iron sulphide? (For iron sulphide, Ksp = 6.3 × 10–18).
How do you count for the following observations?
  1. Though alkaline potassium permanganate and acidic potassium permanganate both are used as oxidants, yet in the manufacture of benzoic acid from toluene we use alcoholic potassium permanganate as an oxidant. Why? Write a balanced redox equation for the reaction.
  2. When concentrated sulphuric acid is added to an inorganic mixture containing chloride, we get colourless pungent smelling gas HCl, but if the mixture contains bromide then we get red vapour of bromine. Why?
Equal volumes of 0.002 M solutions of sodium iodate and cupric chlorate are mixed. Will it lead to the precipitation of copper iodate? (For cupric iodate Ksp = 7.4 × 10–8).
What is the wavelength of light emitted when the electron in a hydrogen atom undergoes transition from an energy level with n = 4 to an energy level with n = 2?
Electromagnetic radiation of wavelength 242 nm is just sufficient to ionize the sodium atom. Calculate the ionization energy of sodium in kJ mol–1.
What sorts of information can you draw from the following reaction?
(CN)2(g)+2OH-(aq)\(\rightarrow\)CN-(aq)+CNO-(aq)+H2O(l)
A photon of wavelength \(4×10^{–7} m\) strikes on metal surface, the work function of the metal being \(2.13\ eV\). Calculate:
  1. the energy of the photon (eV)
  2. the kinetic energy of the emission, and
  3. the velocity of the photo electron (\(1\ eV= 1.6020×10^{–19} J\)).
Determine the solubilities of silver chromate, barium chromate, ferric hydroxide, lead chloride and mercurous iodide at 298K from their solubility product constants.Determine also the molarities of individual ions.
Consider the elements: \(Cs\), \(Ne\), \(I\) and \(F\)
  1. Identify the element that exhibits only negative oxidation state. 
  2. Identify the element that exhibits only postive oxidation state. 
  3. Identify the element that exhibits both positive and negative oxidation states.
  4. Identify the element which exhibits neither the negative nor does the positive oxidation state.
Refer to the periodic table given in your book and now answer the following questions:
  1. Select the possible non metals that can show disproportionation reaction. 
  2. Select three metals that can show disproportionation reaction
Predict the products of electrolysis in each of the following:
  1. An aqueous solution of \(AgNO_3\) with silver electrodes
  2. An aqueous solution \(AgNO_3\) with platinum electrodes 
  3. A dilute solution of \(H_2SO_4\) with platinum electrodes 
  4. An aqueous solution of \(CuCl_2\) with platinum electrodes.
Calculate the wavelength, frequency and wave number of a light wave whose period is \(2.0×10^{–10}\ s\).
What is the number of photons of light with a wavelength of 4000 pm that provide 1J of energy?