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

Total Questions: 41

11. The number of molecules present in 88 g of CO₂ is : [RRB Group D 12/10/2018 (Morning)]

Correct Answer: (a) 2
Solution:

12. The formula unit mass of CaCl₂ is: [RRB Group D 15/10/2018 (Afternoon)]

Correct Answer: (b) 111 u
Solution:

Atomic mass calcium (Ca) = 40 u,
Atomic mass of chlorine (Cl) = 35.5 u,
number of atoms of calcium (Ca) = 1,
number of atoms of chlorine (Cl) = 2.
Unit mass = (atomic mass of calcium × number of atoms of calcium ) + (Atomic mass of chlorine × number of atoms of chlorine).
Unit mass = (40 × 1) + (35.5 × 2) = 111 u.

13. 16 g oxygen contains_______ atoms of oxygen. [RRB Group D 15/10/2018 (Afternoon)]

Correct Answer: (b) 6.022 × 10²³
Solution:

14. What is the molecular mass of ozone ? [RRB Group D 16/10/2018 (Morning)]

Correct Answer: (b) 48 u
Solution:

Ozone (O₃) consists of three oxygen atoms (O).
The atomic mass of oxygen is = 16 u
So, the molecular mass of ozone (O₃) is = 3 × 16u = 48 u.

15. The formula unit mass of CaO is- [RRB Group D 16/10/2018 (Morning)]

Correct Answer: (d) 56
Solution:

The atomic masses of calcium (Ca) = 40.08 u and oxygen (O) = 16.00 u
The formula unit mass of CaO is the sum of the atomic masses of Ca and O:
= 40.08 u + 16.00 u = 56.08 u ≈ 56 u.

16. What is the mass of 10 moles of C𝑂₂ ? [RRB Group D 22/10/2018 (Morning)]

Correct Answer: (a) 440
Solution:

The atomic mass of carbon (C) = 12 g/mol, and the atomic mass of oxygen (O) = 16.00 g/mol.
Molar mass of CO₂
= 1 × 12 + 2 × 16 = 44 g/mol.
∴ The mass of 10 moles : Mass
= Number of moles × Molar mass
= 10 × 44 = 440 g.

17. Molecular mass of CO is ______. [RRB Group D 22/10/2018 (Afternoon)]

Correct Answer: (d) 28
Solution:

Mass of 1 mole = 6.022×10²³ gm.
Mass of carbon monoxide (CO) = 12 + 16 = 28 u.

18. There are 3 moles of sodium in 69 grams of sodium. What is the atomic mass of sodium? [RRB Group D 23/10/2018 (Evening)]

Correct Answer: (c) 23
Solution:

Given, Mass of Sodium (Na) = 69 grams, Number of moles = 3
Number of moles =  𝐺𝑖𝑣𝑒𝑛 𝑀𝑎𝑠𝑠/𝐺𝑟𝑎𝑚 𝐴𝑡𝑜𝑚𝑖𝑐 𝑀𝑎𝑠𝑠
Atomic mass of Sodium = 69/3 = 23 g.

19. What is the mass of 5 water molecules ? [RRB Group D 26/10/2018 (Afternoon)]

Correct Answer: (b) 90 unit
Solution:

Mass of one mole of water molecule = 18 g Then, the mass of 5 moles of water molecules = 5 ×18 = 90 units.

20. 220 g of carbon dioxide contains ____________ moles. [RRB Group D 29/10/2018 (Morning)]

Correct Answer: (a) 5
Solution:

One mole of Carbon dioxide contains one part carbon, two parts Oxygen.
The Molar mass of carbon is 12 g and oxygen is 16 g.
So, 1 mole of CO₂ weighs : (16 × 2) g
= 32 g for oxygen, 12 g for carbon.
The molar mass of CO₂ = 32 + 12 = 44 g.
The number of moles of CO₂ in 220 g of
CO₂ = 220/44 = 5 moles.