Class 11th

This is a Multiple Choice Questions as classified in NCERT Exemplar

C

Boric acid is Lewis acid, having six electrons in its valence shell. It combines with water, accepts electrons

from OH⁻of water molecule and complete it octet to 8 and releases H⁺

Reaction: B (OH)₃ + OH − H→ [B (OH)₄]⁻ + H⁺

This is a short answer type question as classified in NCERT Exemplar

Let b be the position vector of the centre of mass of a regular n-polygon.

(n-1) equal point masses are placed at (n-1) vertices of the regular n polygon, therefore for its centre of mass

r_CM= $\frac{\left(n-1\right)mb+ma}{\left(n-1\right)m+m}=0$

(n-1)mb+ma=0

B=- $\frac{a}{n-1}$

This is a Multiple Choice Questions as classified in NCERT Exemplar

A

Number of items/molecules bonded to the central atom is termed as coordination number. In the given MF₆³⁻

Coordination number of metal is six and boron can have topmost coordination number of four as it consists of s and p orbital only and lacks d-orbital. Therefore, boron cannot be compared in the form of MF₆³⁻.

This is a Multiple Choice Questions as classified in NCERT Exemplar

A

The B₂O₃ reacts with water to form boric acid hence it is an acidic oxide. As we move down the group electronegativity decreases and the tendency to donate electrons increases therefore basic character increases or acidic character decreases. So, from the given options, the most acidic oxide is B₂O₃.

This is a short answer type question as classified in NCERT Exemplar

Where weight of the door acts along negative y axis

A force can produce torque only along a direction normal to itself as $\tau =r*F$ . So when the door is in the xy plane the torque produced by gravity can only along z direction. Never about an axis passing through y direction. Hence the weight will not produce any torque.

This is a Multiple Choice Questions as classified in NCERT Exemplar

A

Central atom in [B (OH)₄ ]⁻ no lone pair. Electronic conf In ground state: 1s² 2s² 2px¹ 2py⁰ 2pz⁰

In excited state, One Electron from 2s shifts to 2py  orbital and the configuration becomes:

1s² 2s² 2p¹ x² p¹ y² pz⁰ 

Now, one s and three p orbitals combined to give sp³ hybridisation and tetrahedral shape.

This is a Multiple Choice Questions as classified in NCERT Exemplar

A

Lewis acids are the species in which the state is not complete and ready to accept electrons. Because Al is surrounded by 6 electrons in AlCl₃ and all three Cl atoms are surrounded by 8 electrons, AlCl₃ is an electron acceptor. It is a covalent compound.

This is a short answer type question as classified in NCERT Exemplar

Wheel is a rigid body. The particles that constitute the wheel do experience a centripetal acceleration directed towards the centre. This acceleration arises due to internal elastic forces which cancel out in pairs.

In a half wheel the distribution of mass about its centre of mass is not symmetrical, therefore the direction of angular momentum of the wheel does not coincide with the direction of its angular velocity. Hence an external torque is required to maintain the motion of the wheel.

This is a Multiple Choice Questions as classified in NCERT Exemplar

C

Gallium generally exists as solid at room temperature but melts on slight heating (melting point: 20^? C ).

Whereas the boiling point of Gallium is very high around 2400^? C. Gallium has large cohesive forces that hold its structure together and it is stable for a wide range of temperatures and can be used for measuring high temperatures.