List of top Chemistry Questions

Symbols \(_{79}^{35}\textrm{Br}\) and \(_{79}^{}\textrm{Br}\) can be written, whereas symbols \(_{35}^{79}\textrm{Br}\) and \(_{35}^{}\textrm{Br}\) are not acceptable. Answer briefly.
An element with mass number 81 contains 31.7% more neutrons as compared to protons. Assign the atomic symbol.
An ion with mass number 56 contains 3 units of positive charge and 30.4% more neutrons than electrons. Assign the symbol to this ion.
Arrange the following types of radiations in increasing order of frequency: 
(a) radiation from the microwave oven 
(b) amber light from a traffic signal 
(c) radiation from FM radio 
(d) cosmic rays from outer space and (e) X-rays.
Nitrogen laser produces radiation at a wavelength of 337.1 nm. If the number of photons emitted is 5.6 × 1024, calculate the power of this laser.
Neon gas is generally used in the sign boards. If it emits strongly at 616 nm, calculate 
(a) the frequency of emission
(b) distance traveled by this radiation in 30 s 
(c) the energy of quantum and 
(d) number of quanta present if it produces 2 J of energy.
The first\((\triangle_ iH_1)\) and the second \((\triangle_iH)\) ionization enthalpies (in \(kJ\; mol ^{-1}\) ) and the \((\triangle_{eg}H)\) electron gain enthalpy (in \(kJ \;mol^{ -1}\) ) of a few elements are given below: 
Elements                       \(\triangle_iH\)                \(\triangle_iH\)                \(\triangle_{eg}H\) 
   I                                    520                   7300                    -60 
  II                                    419                   3051                    -48 
  III                                  1681                  3374                    -328  
  IV                                  1008                 1846                    -295 
  V                                   2372                  5251                   +48 
  VI                                  738                    1451                    -40
Which of the above elements is likely to be : 
  1. the least reactive element.
  2. the most reactive metal.
  3. the most reactive non-metal.
Use the periodic table to answer the following questions.
  1. Identify an element with five electrons in the outer subshell.
  2. Identify an element that would tend to lose two electrons. 
  3. Identify an element that would tend to gain two electrons.
  4. Identify the group having metal, non-metal, liquid as well as gas at the room temperature.
The increasing order of reactivity among group 1 elements is \(Li\) \(<Na\) \(<K\) \(<Rb\) \(<Cs\) 
whereas that among group 17 elements is \(F>Cl>Br>I\) Explain.
Assign the position of the element having outer electronic configuration
  1. \(ns^ 2 np^ 4\) for \(n\) = \(3\) 
  2. \((n - 1)d^ 2 \;ns^ 2\) for \(n\) =\( 4\), and 
  3. \((n - 2) f ^7 (n - 1)d ^1 ns^ 2\) for \(n\) = \(6\), in the periodic table.
Predict the formula of the stable binary compounds that would be formed by the combination of the following pairs of elements.
  1. Lithium and oxygen 
  2. Magnesium and nitrogen 
  3. Aluminium and iodine 
  4. Silicon and oxygen
  5. Phosphorus and fluorine 
  6. Element 71 and fluorine
A liquid is in equilibrium with its vapour in a sealed container at a fixed temperature. The volume of the container is suddenly increased.
  1. What is the initial effect of the change on vapour pressure?
  2. How do rates of evaporation and condensation change initially?
  3. What happens when equilibrium is restored finally and what will be the final vapour pressure?
In the modern periodic table, the period indicates the value of:
Which of the following statements related to the modern periodic table is incorrect?
What is Kc for the following equilibrium when the equilibrium concentration of each substance is: [SO2]= 0.60 M, [O2] = 0.82 M and [SO3] = 1.90 M ?
\(2SO_2(g) + O_2(g) ⇋ 2SO_3(g)\)
Which one of the following statements is incorrect in relation to ionization enthalpy?
Considering the elements B, Al, Mg, and K, the correct order of their metallic character is:
Considering the elements B, C, N, F, and Si, the correct order of their non-metallic character is:
At a certain temperature and total pressure of 105 Pa, iodine vapour contains 40% by volume of \(\text {I}\) atoms
\( I_2 (g) ⇋ 2I (g)\)
Calculate Kp for the equilibrium.
Considering the elements F, Cl, O and N, the correct order of their chemical reactivity in terms of oxidizing property is:
Write the expression for the equilibrium constant, Kc for each of the following reactions: 
  1. \(2NOCl (g) ⇋ 2NO (g) + Cl_2 (g)\)
  2. \(2Cu(NO_3)_2 (s) ⇋ 2CuO (s) + 4NO_2 (g) + O_2 (g)\)
  3. \(CH_3COOC_2H_5(aq) + H_2O(l) ⇋ CH_3COOH (aq) + C_2H_5OH (aq)\)
  4. \(Fe^{3+} (aq) + 3OH^– (aq) ⇋ Fe(OH)_3 (s)\)
  5. \(I_2 (s) + 5F_2 ⇋ 2IF_5\)
Define octet rule. Write its significance and limitations.
Discuss the shape of the following molecules using the VSEPR model: 
BeCl2, BCl3, SiCl4, AsF5, H2S, PH3
Although geometries of NH3 and H2O molecules are distorted tetrahedral, bond angle in water is less than that of ammonia. Discuss.
Explain the important aspects of resonance with reference to the \(CO_3^{2-}\) ion.