List of top Chemistry Questions

(i) Calculate the number of electrons which will together weigh one gram.
(ii) Calculate the mass and charge of one mole of electrons.
Convert the following into basic units:
  1. 28.7 pm
  2. 15.15 pm
  3. 25365 mg
How would you explain the fact that the first ionization enthalpy of sodium is lower than that of magnesium but its second ionization enthalpy is higher than that of magnesium?
Among the second period elements the actual ionization enthalpies are in the order 
Li < B < Be < C < O < N < F < Ne. 
Explain why 
  1. Be has higher ΔiH than B
  2. O has lower ΔiH than N and F?
Energy of an electron in the ground state of the hydrogen atom is -2.18 x 10-18  J. Calculate the ionization enthalpy of atomic hydrogen in terms of J mol-1  .
In a reaction \(A + B_2 → AB_2\) Identify the limiting reagent, if any, in the following reaction mixtures.
  1. 300 atoms of A + 200 molecules of B
  2. 2 mol A + 3 mol B
  3. 100 atoms of A + 100 molecules of B
  4. 5 mol A + 2.5 mol B
  5. 2.5 mol A + 5 mol B
If the speed of light is 3.0×108 ms–1, calculate the distance covered by light in 2.00 ns.
What is the significance of the terms - 'isolated gaseous atom' and 'ground state' while defining the ionization enthalpy and electron gain enthalpy?
Hint: Requirements for comparison purposes.
Would you expect the second electron gain enthalpy of O as positive, more negative or less negative than the first? Justify your answer.
What is the basic difference between the terms electron gain enthalpy and electronegativity?
How would you react to the statement that the electronegativity of N on Pauling scale is 3.0 in all the nitrogen compounds?
Describe the theory associated with the radius of an atom as it 
  1. gains an electron 
  2. loses an electro
Would you expect the first ionization enthalpies for two isotopes of the same element to be the same or different? Justify your answer.

(i) Calculate the total number of electrons present in one mole of methane.
(ii) Find (a) the total number and (b) the total mass of neutrons in 7 mg of 14C. (Assume that the mass of a neutron = 1.675 × 10–27 kg).
(iii) Find (a) the total number and (b) the total mass of protons in 34 mg of NH3 at STP. Will the answer change if the temperature and pressure are changed?

Explain the formation of a chemical bond.
Write Lewis dot symbols for atoms of the following elements: Mg, Na, B, O, N, Br.
Which one of the following will have the largest number of atoms?
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 minimum volume of water required to dissolve 1g of calcium sulfate at 298 K? (For calcium sulfate, Ksp is 9.1 × 10–6).
What is the maximum number of emission lines when the excited electron of a H atom in n = 6 drops to the ground state?
Why does the following reaction occur?
\(XeO_6^{4-}(aq) + 2F^ - (aq) + 6H^ + (aq) \rightarrow XeO_3(g) + F_2(g) + 3H_2O(l)\) What conclusion about the compound \(Na_4XeO_6\)(of which \(XeO_6^{4-}\) is a part) can be drawn from the reaction.
  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.
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?
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).