Science (English Medium) इयत्ता १२ - CBSE Important Questions for Chemistry

Give reasons : Silicon on doping with Phosphorous forms n-type semiconductor.

An element with density 11.2 g cm^–3 forms a f.c.c. lattice with edge length of 4 × 10^–8 cm.
Calculate the atomic mass of the element.
(Given : N_A = 6.022 × 10²³ mol^–1)

Examine the given defective crystal:

Answer the following questions :
(i) What type of stoichiometric defect is shown by the crystal?
(ii) How is the density of the crystal affected by this defect?
(iii) What type of ionic substances show such defect?

Calculate the mass of a compound (molar mass = 256 g mol⁻¹) to be dissolved in 75 g of benzene to lower its freezing point by 0.48 K (K_f = 5.12 K kg mol⁻¹).

An unripe mango placed in a concentrated salt solution to prepare pickle, shrivels because ______.

When 19.5 g of F – CH₂ – COOH (Molar mass = 78 g mol⁻¹), is dissolved in 500 g of water, the depression in freezing point is observed to be 1°C. Calculate the degree of dissociation of F – CH₂ – COOH.

[Given: K_f for water = 1.86 K kg mol⁻¹]

Why is the value of van't Hoff factor for ethanoic acid in benzene close to 0.5?

Why is boiling point of 1 M NaCl solution more than that of 1 M glucose solution?

A non-volatile solute 'X' (molar mass = 50 g mol₋₁), when dissolved in 78 g of benzene, reduced its vapour pressure to 90%. Calculate the Mass of X dissolved in the solution.

Calculate the boiling point elevation for a solution prepared by adding 10 g of MgCl₂ to 200 g of water, assuming MgCl₂ is completely dissociated.

(K_b for Water = 0.512 K kg mol⁻¹, Molar mass MgCl₂ = 95 g mol⁻¹)

Determine the osmotic pressure of a solution prepared by dissolving 2.32 × 10⁻² g of K₂SO₄ in 2L of solution at 25°C assuming that K₂SO₄ is completely dissociated.

(R = 0.082 L atm K⁻¹ mol, Molar mass K₂SO₄ = 174 g mol⁻¹)

When 25.6 g of sulphur was dissolved in 1000 g of benzene, the freezing point lowered by 0.512 K. Calculate the formula of sulphur (Sr).

(K_f for benzene = 5.12 K kg mol⁻¹, Atomic mass of sulphur = 32 g mol⁻¹)

From the given cells:

Lead storage cell, Mercury cell, Fuel cell and Dry cell

Answer the following:

(i) Which cell is used in hearing aids?

(ii) Which cell was used in Apollo Space Programme?

(iii) Which cell is used in automobiles and inverters?

(iv) Which cell does not have long life?

Why conductivity of an electrolyte solution decreases with the decrease in concentration ?

Calculate the mass of Ag deposited at cathode when a current of 2 amperes was passed through a solution of AgNO₃ for 15 minutes.

(Given : Molar mass of Ag = 108 g mol⁻¹ lF = 96500 C mol⁻¹)

Define the following terms :

Limiting molar conductivity

Read the passage given below and answer the questions that follow:

1. Is silver plate the anode or cathode?  (1)
2. What will happen if the salt bridge is removed?  (1)
3. When does electrochemical cell behaves like an electrolytic cell?  (1)
4. (i) What will happen to the concentration of Zn²⁺ and Ag⁺ when E_cell = 0.   (1)
   (ii) Why does conductivity of a solution decreases with dilution?  (1)
   OR
   The molar conductivity of a 1.5 M solution of an electrolyte is found to be 138.9 S cm²mol^-1. Calculate the conductivity of this solution.  (2)

Conductivity of 2 × 10⁻³ M methanoic acid is 8 × 10⁻⁵ S cm⁻¹. Calculate its molar conductivity and degree of dissociation if `∧_"m"^0` for methanoic acid, is 404 S cm² mol⁻³.

Calculate the Δ_rG⁰ and log K_c, for the given reaction at 298 K:

\[\ce{Ni_{(s)} + 2Ag^+_{( aq)} <=> Ni^{2+}_{( aq)} + 2Ag_{(s)}}\]

Given: `"E"_("Ni"^(2+)//"Ni")^0` = −0.25 V, `"E"_("Ag"^+//"Ag")^0` = +0.80 V, 1F = 96500 C mol⁻¹.

Four half-reactions, I to IV are shown below:

1. \[\ce{2Cl^- -> Cl2 + 2e^-}\]
2. \[\ce{4OH^- -> O2 + 2H2O + 2e^-}\]
3. \[\ce{Na^+ + e^- -> Na}\]
4. \[\ce{2H^+ + 2e^- -> H2}\]

Which two of these reactions are most likely to occur when concentrated brine is electrolysed?