Subject

Chemistry

Class

CBSE Class 12

Pre Boards

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Sample Papers

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 Multiple Choice QuestionsShort Answer Type

1.

Following reactions occur at cathode during the electrolysis of aqueous silver chloride solution:


Ag+ (aq) + e-  →  Ag(s)          E° = +0.80 V

 H+ (aq) + e-   → 1/2 H2 (g)     E° = 0.00 V

 On the basis of their standard reduction electrode potential (E°) values, which reaction is feasible at the cathode and why?


We have given:

 Ag+ (aq) + e- → Ag(s)          E° = +0.80 V

 H+ (aq) + e-→ 1/2 H2 (g)      E° = 0.00 V

 

The relationship between the standard free energy change and emf of a cell reaction is given by 
∆ G = – nFE(cell)

Thus, the more positive the standard reduction potential of a reaction, the more negative is the standard free energy change associated with the process and, consequently, the higher is the feasibility of the reaction.
Since E 0 Ag+/Ag has a greater positive value than E0 H+ /H, the reaction which is feasible at the cathode is given by

 Ag+ (aq) + e- → Ag(s)         

 

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2.

What is the formula of a compound in which the element Y forms ccp lattice and atoms of X occupy 1/3rd of tetrahedral voids?


Number of tetrahedral voids formed = 2 x Number of atoms of element Y

Number of atoms of element Y in the ccp unit cell = 4

Number of tetrahedral voids formed = 2 x 4 = 8


Number of tetrahedral voids occupied by atoms of X = 1/3x8

Ratio of the numbers of atoms of X and Y = 8/3: 4 =2: 3


Hence, the formula of the compound is X2Y3.

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3.

Define limiting molar conductivity. Why conductivity of an electrolyte solution decreases with the decrease in concentration?


The limiting molar conductivity of an electrolyte is defined as its molar conductivity when the concentration of the electrolyte in the solution approaches zero.

The conductivity of an electrolyte solution is the conductance of ions present in a unit volume of the solution. The number of ions (responsible for carrying current) decreases when the solution is diluted or the concentration is decreased. As a result, the conductivity of an electrolyte solution decreases with the decrease in concentration.

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4.

 What is meant by positive deviations from Raoult's law? Give an example. What is the sign of mixH for positive deviation?

OR

Define azeotropes. What type of azeotrope is formed by positive deviation from Raoult's law? Give an example.


The solutions that do not obey Raoult’s law over the entire range of concentration are known as non-ideal solutions. They have vapour pressures either higher or lower than those predicted by Raoult’s law. If the vapour pressure is higher, then the solution is said to exhibit a positive deviation from Raoult’s law.

 A mixture of ethanol and acetone is an example of a solution that shows a positive deviation from Raoult’s law.

 In case of solutions showing positive deviations, absorption of heat takes place that  mixH has a positive sign.

 OR

Azeotropes are the binary mixtures of solutions that have the same composition in liquid and vapour phases and that have constant boiling points.

 A minimum-boiling azeotrope is formed by solutions showing a large positive deviation from Raoult’s law at a specific composition.

 Example: An ethanol–water mixture containing approximately 95% ethanol by volume

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5.

Write the formula for the following complex:


Pentaamminenitrito-o-Cobalt (III)


Formula: [Co(NH3)5(ONO)]2+

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6.

Out of BaCl2 and KCl, which one is more effective in causing coagulation of a negatively charged colloidal Sol? Give reason.


According to the Hardy–Schulze rule, the greater the valency of a flocculating ion, the greater will its coagulating power. In Between Ba2+ (from BaCl2) and K+ (from KCl), Ba2+ has greater valency 2 whereas KCl has valency 1. Therefore, BaCl2 will be more effective in causing the coagulation of a negatively charged colloidal sol.

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7.

What are the transition elements? Write two characteristics of the transition elements ?


Elements that possess incompletely filled d-orbitals either in their ground state or in any of their oxidation states are known as transition elements. The name transition element given to the elements of d-block is only because of their position between s-block and p-block elements.

Characteristics of transition elements are as follows:


1. They show paramagnetic behaviour.

2. They show variable oxidation states.

3. They exhibit catalytic properties.

4. They generally form complex compounds.

5. They generally form coloured compounds.

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8.

What is the basicity of H3PO4?


The basicity is defined as the number of hydrogen atom replaceable by a base in a particular acid.  H3PO4 has three ionizable hydrogen atoms.

 Hence, its basicity is 3.

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9.

Write down the IUPAC name of the following complex:


[Cr(NH3)2Cl2(en)]Cl    (en = ethylenediamine)


Cr(NH3)2Cl2(en)]Cl


IUPAC Name: Diamminedichlorideethyldiamine chromium (III) chloride

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10.

Draw the geometrical isomers of complex [Pt(NH3)2Cl2]? 


Geometrical isomers of complex [Pt(NH3)2Cl2]



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