Chemistry 12th Important questions and answers 2019
1. Describe how electrical neutrality is maintained in compounds showing Frenkel and Schottky defect?
Ans: In compound showing Frenkel defect, ions just get displaced within the lattice. While incompounds showing Schottky defect, equal number of anions andcations are removed from the lattice. Thus, electrical neutrality ismaintained in both cases.
2. Calculate the number of atoms in a cubic unit cell having one atom on each corner and two atoms on each body diagonal?
Ans: 8 corner atoms × 1/8 atom per unit cell = 1 atom There are four body diagonals in a cubic unit cell and each has two body centre atoms. So 4×2=8 atoms therefore total number of atoms per unit cell = 1+8=9 atoms.
3. The electrical conductivity of a metal decreases with rise in temperature while that of a semi-conductor increases.Explain?
Ans: In metals with increase of temperature, the kernels start vibrating and thus offer resistance to the flow of electrons.Hence conductivity decreases. In case of semiconductors, with increase of temperature, more electrons can shift from valence band to conduction band. Hence conductivity increases.
4. What type of substances would make better permanent magnets, ferromagnetic or ferromagnetic,Why?
Ans: Ferromagnetic substances make better permanent magnets. This is because the metal ions of a ferromagnetic substance are grouped into small regions called domains. Each domain acts as tiny magnet and get oriented in the direction of magnetic field in which it is placed. This persists even in the absence of magnetic field.
5. In a crystalline solid, the atoms A and B are arranged as follows:
a. atoms A are arranged in ccp array
b. atoms B occupy all the octahedral voids and half of the tetrahedral voids. What is the
formula of the compound?
Ans: Let no. of atoms of A be N, No. of octahedral voids = N, No. of tetrahedral voids = 2N,
i)There will be one atom of B in the octahedral void,
ii)There will be one atom of B in tetrahedral void (1/2 X 2N), Therefore, total 2 atoms of B for each atom of A Therefore formula of the compound = AB2.
6. What type of substances would make better permanent magnets ferromagnetic or ferromagnetic ? Why ?
Ans: Ferromagnetic substances make better permanent magnets. This is because the metal ions of a ferromagnetic substance are grouped into small regions called domains. Each domain acts as tiny magnet and get oriented in the direction of magnetic eld in which it is placed. This persists even in the absence of magnetic field.
7. The electrical conductivity of a metal decreases with rise in temperaturewhile that of a semi-conductor increases. Explain?
Ans: In metals with increase of temperature, the kernels start vibrating at faster rate and thus offer resistance to the ow of electrons. Hence, conductivity decreases. In case of semi-conductors, with increase of temperature, more electrons can shift from valence band to conduction band. Hence conductivity increases.
8. Explain how electrical neutrality is maintained in compounds showingFrenkel and Schottky defect?
Ans: In compound showing Frenkel defect, ions just get displaced within the lattice, while in compounds showing Schottky defect, equal number of anions and cations are removed from the lattice. Thus, electrical neutrality is maintained in both cases.
9. Calculate the number of atoms in a cubic unit cell having one atom on each corner and two atoms on each body diagonal?
Ans: 8 corner × 1/8 atom per unit cell = 1 atom ,there are four body diagonals in a cubic unit cell and each has two body centre atoms.
So, 4 × 2 = 8 atoms therefore, total number of atoms per unit cell = 1 + 8 = 9.
10. Classify each of the following as either a p-type or n-type semi-conductor :
(a) Ge doped with In
(b) B doped with Si
Ans: (a) Ge is group 14 element and In is group 13 element. Therefore, an electron decit hole is created. Thus semi-conductor is p-type.
(b) Since B is group 13 element and Si is group 14 element, there will be a free electron, thus, it is n-type semi-conductor.
11. Why is liquid ammonia bottle first cooled in ice before opening it ?
Ans: At room temperature, the vapour pressure of liquid ammonia is very high. On cooling vapour pressure decreases, therefore the liquid ammonia will not splash out.
12. Which colligative property is preferred for the molar mass determination of macromolecules ?
Ans: Osmotic pressure measurement is preferred for molar mass determination because :
(a) even in dilute solution the osmotic pressure values are appreciably high and can be measured accurately.
(b) osmotic pressure can be measured at room temperature.
13. Define conductivity and molar conductivity for a solution of an electrolyte.?
Ans: Conductivity is dened as ease with which current ows through electrolyte. It is reciprocal of specic resistance. Molar conductivity is conductance of all the ions produced by one mole of electrolyte when electrodes are at unit distance apart and have sufcient area of cross-section to hold electrolyte.
14. State the following:
i) Raoult’s Law in its general form in reference to solutions.
ii) Vant Hoff factor.
Ans: i) For a solution of volatile solute the partial vapour pressure of each component in the solution is directly proportional to its mole fraction. For a solution of a non- volatile solute the vapour pressure of the solution is directly proportional to the mole fraction of the solvent.
ii) Vant Hoff factor is defined as the ratio of theoretical molecular mass to the observed molecular mass of the solute on solution.
15. i) Why is an increase in temperature observed on mixing chloroform and acetone?
ii) Why does sodium chloride solution freeze at a lower temperature than water?
Ans: i) When chloroform and acetone are mixed , the new interactions are stronger i.e. ΔH is negative i.e. heat is evolved and hence the escaping tendency of the molecules from the mixture decreases.
ii) Upon adding a non- volatile solute like NaCl to water, the vapour pressure is lowered. The vapour pressure of this solution becomes equal to that of pure solid solvent which is the freezing point of the solution occurs at a lower temperature. Thus freezing point of NaCl solution is lower than that of pure solvent.
16. Which of the two molality and molarity is better to express concentration of solution? Why?
Ans: Molality does not change with temperature where as molarity changes with temperature because volume changes with temperature. Therefore molality is better.
17. Give an example of a miscible liquid pair showing positive deviation from the Raoult’s Law.
Ans: An example of a miscible liquid pair showing positive deviation from the Raoult’s Law is a mixture of Ethyl alcohol and cyclohexane
When cyclohexane is added to ethyl alcohol, the molecules of cyclohexane try to occupy the spaces in between ethyl alcohol molecules. Consequently some hydrogen bonds in alcohol molecules break and attractive forces between the molecules are weakened . The escaping tendency of the molecules of ethanol and cyclohexane from the solution increases due to which the vapour pressure of the solution also increases.
18. Differentiate between molarity and molality of a solution. Explain the effect of change in temperature on its molality and molarity values?
Ans: Molality of a solution is the number of moles of solute present per litre of the solution and it is influenced by temperature change since volume increases with rise in temp and vice versa. Molarity decreases with increase in temperature . On the other hand ,molality of a solution is the number of moles of the solute present per Kg of the solvent and it does not change with temperature.
19. What is the effect of rise in temperature on solubility of gases?
Ans: Dissolution of gas is exothermic process. Hence according to LeChatelier‘s principle, the solubility of gas should decrease with rise in temperature.
20. Define the terms osmosis and osmotic pressure. Is the osmotic pressure of a solution a colligative property?
Ans: Osmosis : It is the process of movement of solvent from the solvent to the solution. Colligative properties are properties of solutions that depend on the number of particles in a volume of solvent and not on the mass of the particles. Since osmotic pressure depends upon the number of solute particles in a given amount of solvent, osmotic pressure is a colligative property.