Railway Science (Chemistry-Structure of Atom) (Part-III)

Total Questions: 41

1. Calculate the relative molecular mass of water (H₂O). [RRB Group D 03/10/2018 (Morning)]

Correct Answer: (a) 18 u
Solution:

Atomic mass of Hydrogen = 1u,
Atomic mass of Oxygen = 16u
Relative Molecular mass of water (H₂O)
= 2(1) + 16 = 18u.

2. ____ is the molecular mass of H₂SO₄ [RRB Group D 04/10/2018 (Afternoon)]

Correct Answer: (d) 98 u
Solution:

Molecular mass of  H₂SO₄ = 1 × 2 (2 atoms of Hydrogen) + 32 × 1 (1 atom of Sulphur) + 16 × 4 (4 atoms of Oxygen) = 2 + 32 + 64 = 98 u.

3. Atomic mass of carbon is 12. The number of moles in 24 g of carbon is: [RRB Group D 05/10/2018 (Morning)]

Correct Answer: (d) 2
Solution:

The atomic mass of carbon is 12g. This means that 12 g of carbon contains 1 mole of carbon atoms. Hence, 24 grams of carbon contains 2 moles of carbon atoms.

4. he electronic configuration of an element is 2, 8, 5, then find its number of protons ? [RRB Group D 05/10/2018 (Afternoon)]

Correct Answer: (b) 15
Solution:

Total no. of electrons = 2 + 8 + 5 = 15.
Number of electrons in an atom = Number of protons = Atomic Number = 15.

5. Chlorine gas is a mixture of two isotopes of chlorine whose ratio is 3 : 1. Which of the following are these two isotopes ? [RRB Group D 08/10/2018 (Morning)]

Correct Answer: (a) ³⁵CI and ³⁷CI
Solution:

6. The number of moles present in 90 g of water is ________. [RRB Group D 09/10/2018 (Afternoon)]

Correct Answer: (a) 5
Solution:

7. The relative mass of Ca(OH)₂ is : [RRB Group D 10/10/2018 (Morning)]

Correct Answer: (d) 74 u
Solution:

Atomic mass of Ca (calcium) = 40.08 u
Atomic mass of O (oxygen) = 15.99 u
(there are 2 oxygen atoms)
Atomic mass of H (hydrogen) = 1.008 u
(there are 2 hydrogen atoms)
Total mass = 40.08 u + 31.998 u + 2.016 u = 74.094 u

8. The number of moles present in 90g of water is _________. [RRB Group D 10/10/2018 (Evening)]

Correct Answer: (b) 5
Solution:

Given : Mass (m) = 90 gm The molecular mass of H₂O
= (1 × 2) + (1 × 16) = 18 u
No. of moles = 𝑚𝑎𝑠𝑠 𝑖𝑛 𝑔𝑟𝑎𝑚/𝑚𝑜𝑙𝑒𝑐𝑢𝑙𝑎𝑟 𝑚𝑎𝑠𝑠
No. of moles = 90/18
No. of moles = 5.

9. The relative mass of CO is: [RRB Group D 11/10/2018 (Afternoon)]

Correct Answer: (a) 28
Solution:

Carbon monoxide (CO) has one carbon atom and one oxygen atom.
The atomic mass of carbon (C) is = 12 amu.
The atomic mass of oxygen (O) is = 16 amu.
Therefore, the relative mass of CO is
= 12 amu (C) + 16 amu (O) = 28 amu.

10. In ammonia, the masses of N₂ and H₂ are always in ________ ratio. [RRB Group D 11/10/2018 (Evening)]

Correct Answer: (c) 14 : 3
Solution:

Ammonia has a molecular formula NH₃ .
It is composed of 3 H -atoms and 1 N - atom.
Mass of H-atoms in ammonia = (3 × 1) u.
Mass of N-atom in ammonia = 14u.
Ratio of mass = 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑛𝑖𝑡𝑟𝑜𝑔𝑒𝑛 𝑎𝑡𝑜𝑚/𝑀𝑎𝑠𝑠 𝑜𝑓 ℎ𝑦𝑑𝑟𝑜𝑔𝑒𝑛 𝑎𝑡𝑜𝑚 = 14/3 .