NEW
New Website Launch
Experience the best way to solve previous year questions with mock tests (very detailed analysis), bookmark your favourite questions, practice etc...
1

### JEE Main 2021 (Online) 27th August Evening Shift

Choose the correct statement from the following :
A
The standard enthalpy of formation for alkali metal bromides becomes less negative on descending the group.
B
The low solubility of CsI in water is due to its high lattice enthalpy.
C
Among the alkali metal halides, LiF is least soluble in water.
D
LiF has least negative standard enthalpy of formation among alkali metal fluorides.

## Explanation

a. Standard enthalpy of formation for alkali metal bromides becomes more negative on descending down the group.

b. In case of CsI, lattice energy is less, but Cs+ is having less hydration enthalpy due to which it is less soluble in water.

c. For alkali metal fluorides, the solubility in water increases from lithium to caesium. LiF is least soluble in water.

d. Standard enthalpy of formation for LiF is most negative among alkali metal fluorides.
2

### JEE Main 2021 (Online) 27th August Morning Shift

In which one of the following molecules strongest back donation of an electron pair from halide to boron is expected?
A
BCl3
B
BF3
C
BBr3
D
BI3

## Explanation

Type of back bonding

$$\matrix{ {B{F_3}} & {BCl} & {BB{r_3}} & {B{I_3}} \cr {(2p\pi - 2p\pi )} & {(2p\pi - 3p\pi )} & {(2p\pi - 4p\pi )} & {(2p\pi - 5p\pi )} \cr }$$

Therefore back bonding strength is as follows

BF3 > BCl > BBr3 > BI3
3

### JEE Main 2021 (Online) 26th August Evening Shift

Chalcogen group elements are :
A
Se, Tb and Pu.
B
Se, Te and Po.
C
S, Te and Pm.
D
O, Ti and Po.

## Explanation

Group 16/oxygen family is known as Chalcogens the members are O, S, Se, Te, Po
4

### JEE Main 2021 (Online) 26th August Evening Shift

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R).

Assertion (A) : Barium carbonate is insoluble in water and is highly stable.

Reason (R) : The thermal stability of the carbonates increases with increasing cationic size.
A
Both (A) and (R) are true and (R) is the true explanation of (A)
B
(A) is true but (R) is false
C
Both (A) and (R) are true but (R) is not the true explanation of (A)
D
(A) is false but (R) is true.

## Explanation

In IIA group on moving down the group size of cation increases and show thermal stability of carbonate increases.

### Joint Entrance Examination

JEE Main JEE Advanced WB JEE

### Graduate Aptitude Test in Engineering

GATE CSE GATE ECE GATE EE GATE ME GATE CE GATE PI GATE IN

NEET

Class 12