1

### JEE Main 2019 (Online) 9th January Evening Slot

The temporary hardness of water is due to :
A
Na2SO4
B
NaCl
C
Ca(HCO3)2
D
CaCl2

## Explanation

Ca(HCO3)2 is responsible for temporary hardness of water.
2

### JEE Main 2019 (Online) 10th January Morning Slot

The total number of isotopes of hydrogen and number of radioactive isotopes among them, respectively, are :
A
3 and 2
B
2 and 1
C
2 and 0
D
3 and 1

## Explanation

Total number of isotopes of hydrogen is 3

$\Rightarrow \,\,{}_1^1H,{}_1^2H$  or  ${}_1^2D,\,{}_1^3H\,\,$or  ${}_1^3T$

and only ${}_1^3H$ or ${}_1^3T$ is an Radioactive element.
3

### JEE Main 2019 (Online) 10th January Morning Slot

The chemical nature of hydrogen peroxide is :
A
Oxidising agent in acidic medium, but not in basic medium
B
Oxidising and reducing agent in both acidic and basic medium
C
Reducing agent in basic medium, but not in acidic medium
D
Oxidising and reducing agent in acidic medium, but not in basic medium.

## Explanation

H2O2 act as oxidising agent and reducing agent in acidic medium as well as basic medium.

H2O2 Act as oxidant : -

H2O2 + 2H$^ \oplus$ + 2e$-$ $\to$  2H2O (In acidic medium)

H2O2 + 2e$-$ $\to$ 2OH$-$ (In basic medium)

H2O2 Act as reductant : -

H2O2 $\to$ 2H+ + O2 + 2e$-$ (In acidic medium)

H2O2 + 2OH$-$ $\to$ 2H2O + O2 + 2e$-$ (In basic medium)
4

### JEE Main 2019 (Online) 11th January Morning Slot

The correct statements among (a) to (d) regarding H2 as a fuel are :

(a)   It produces less pollutants than petrol.

(b)   A cylinder of compressed dihydrogen weighs $\sim$ 30 times more than a petrol tank producing the same amount of energy.

(c)   Dihydrogen is stored in tanks of metal alloys like NaNi5.

(d)   On combustion, values of energy released per gram of liquid dihydrogen and LPG are 50 and 142 KJ, respectively.
A
(a) and (c) only
B
(b) and (d) only
C
(a), (b) and (c) only
D
(b), (c) and (d) only