Science (English Medium) Class 12 - CBSE Important Questions

3.9 g of benzoic acid dissolved in 49 g of benzene shows a depression in freezing point of 1.62 K. Calculate the van't Hoff factor and predict the nature of solute (associated or dissociated).

(Given : Molar mass of benzoic acid = 122 g mol⁻¹, K_f for benzene = 4.9 K kg mol⁻¹)

An element with molar mass 27 g mol⁻¹ forms a cubic unit cell with edge length 4.05 ✕ 10⁻⁸ cm. If its density is 2.7 g cm⁻³, what is the nature of the cubic unit cell?

Define the following term:

Schottky defect

An element with density 2.8 g cm^–3 forms a f.c.c. unit cell with edge length 4 x 10^–8 cm. Calculate the molar mass of the element.

(Given : N_A = 6.022 x 10²³ mol ^–1)

Define the following terms : Molarity

Define the following term: 

Molal elevation constant (K_b)

Write any two differences between amorphous solids and crystalline solids

Give reasons : In stoichiometric defects, NaCl exhibits Schottky defect and not Frenkel defect.

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)