Class 11th

5.15. Molar mass of O₂ = 32 g/mol

It means, 8 g of O₂ has 8/32 mol = 0.25 mol

Molar mass of H₂ = 2 g/mol

It means, 4 g of H₂ has 4/2 mol = 2 mol

Therefore, total number of moles, n = 2 + 0.25 = 2.25 mol

Given, V = 1dm³, T = 27°C = 300 K, R = 0.083 bar dm³ K^-1 mol^-1

Applying PV = nRT,

P = nRT / V

   = (2.25) (0.083 bar dm³ K^-1 mol^-1) (300 K) / (1dm³)

   = 56.025 bar

5.14. Time taken to distribute 10¹⁰ wheat grains = 1s

Time taken to distribute Avogadro number of wheat grains = (1s x 6.022 x 10²³) / 10¹⁰

= 6.022 x 10¹³ s

= (6.022 x 10¹³ / 60 x 60 x 24 x 365) year

= 1.9 x 10⁶ years

5.13. Molecular mass of N₂ = 28g
28 g of N₂ has No. of molecules = 6.022 x 10²³ 

1.4 g of N₂ has No. of molecules = (6.022 x 10²³ x 1.4 g)/28 g= 3.011 x 10²² molecules.
Atomic No. of Nitrogen (N) = 7
1 molecule of N₂ has electrons = 7 x 2 = 14
3.011 x 10²² molecules of N₂ have electrons= 14 x 3.011 x 10²²= 4.215 x10²³ electrons.

5.12. Given,

P= 3.32 bar

V= 5 dm³ 

n= 4 mol

R= 0.083 bar dm³ K^-1 mol^-1

PV = nRT

Or T = PV / nR = 3.32 x 5 dm³ / (4.0 mol x 0.083 bar dm³ K^-1 mol^-1)

         = 50 K

9.56. Dihydrogen is prepared from water by the action of alkali metals like Na and K which is a strong reducing agent.

2Na + 2H₂O → 2NaOH + H₂

2K + 2H₂O → 2KOH + H₂

9.55. 1. High heat of combustion.

2. It is pollution free.

9.54. Water which does not produce lathers with soap is known as hard water. Hardness is due to the presence of bicarbonates, sulphates and chlorides of Ca²⁺ and Mg²⁺.

On boiling, the bicarbonates of calcium and magnesium decompose to insoluble carbonate which can be removed by filtration.

9.53. In water molecule, O is SP³ hybridized. Due to stronger lone pair-lone pair repulsion than bond pair-bond pair repulsions, the H-O-H bond angle decreases from 109.5° to 104.5°. Thus, water is bent molecule.

9.52. Many transition and inner-transition metals absorb hydrogen into the interstices of their lattices to yield metal like hydrides also called the interstitial hydrides. These hydrides are generally non stoichiometric and their composition vary with temperature and pressure.
For example, T_iH_1.73, CeH_2.7

Two uses of interstitial hydrides are:

(i) In the storage of H₂.

(ii) Catalyst for hydrogenation reaction.

9.51. Dihydrogen, under certain reaction conditions, combines with almost all elements, except noble gases, to form binary compounds, called hydrides. If 'E' is the symbol of an element then hydride can be expressed as EH_x (e.g., MgH₂) or E_mH_n (e.g., B₂H₆).

The hydrides are classified into three categories:

(i) Ionic or saline or saltlike hydrides

(ii) Covalent or molecular hydrides

(iii) Metallic or non-stoichiometric hydrides