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

Total Questions: 50

41. The mass number of an isotope of an element is 298. If there are 189 neutrons in its nucleus, what will be its atomic number ? [RRB Group D 22/09/2018 (Morning)]

Correct Answer: (d) 109.0
Solution:

The mass number of an element (A) = 298. Number of Neutrons (N) = 189. Also, Number of protons = atomic number (Z).
∵ A = Z + N
Therefore, Z = A − N = 298 - 189 = 109.

42. What is the number of iron atoms in a piece of iron weighing 2.8 g ? [RRB Group D 24/09/2018 (Afternoon)]

Correct Answer: (b) 3.011 × 10²² atoms
Solution:

43. In 23.3 g zinc, the number of moles is __________. [RRB Group D 26/09/2018 (Morning)]

Correct Answer: (a) 0.35
Solution:

Given that, Weight = 23.3 grams Zn. Molar mass of Zn = 65 g/mol.
We know that, Moles = 𝑊𝑒𝑖𝑔ℎ𝑡 𝑖𝑛 𝑔𝑟𝑎𝑚𝑠/𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠
= 23.3/65 =  0.35 moles.

44. The empirical formula of the compound is CH₂O, its vapor density is 90. The molecular formula of the compound is: [RRB Group D 26/09/2018 (Evening)]

Correct Answer: (a) C₆H₁₂O₆
Solution:

45. What is the percentage of oxygen in AI₂(SO₄)₃ ? [RRB Group D 27/09/2018 (Morning)]

Correct Answer: (b) 56.1%
Solution:

46. The atomic number of an element is 13. How many shells are present in this element ? [RRB Group D 27/09/2018 (Evening)]

Correct Answer: (b) 3
Solution:

The electronic configuration of an element with atomic number 13 is 2, 8, 3 and this element is Aluminium (Al). This means that 2 electrons are present in the K shell, 8 electrons in the L shell, and 3 electrons in the M shell. So the total number of electronic shells present in Aluminium (Al) = 3.

47. _____moles are present in 52g of He. [RRB Group D 27/09/2018 (Evening)]

Correct Answer: (c) 13
Solution:

The molar mass of helium gas = 4g/mol.
Given the mass of substance = 52g
Number of Moles =  𝑚𝑎𝑠𝑠 𝑜𝑓 𝑠𝑢𝑏𝑠𝑡𝑎𝑛𝑐𝑒/𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 = 52/4 = 13.

48. One molecule of any substance contains 6.023 × 10²³ particles. If 3.0115 × 10²³ particles are present in CO₂ , then the number of molecules of CO₂ is : [RRB Group D 28/09/2018 (Evening)]

Correct Answer: (a) 0.5
Solution:

We know that one mole of a substance contains 6.023 × 10²³ particles.
Molecular mass of CO₂ = 12 + (16 × 2) = 44g = 1 mole.
Given, 3.0115 × 10²³ particles are present in CO₂
Number of moles in 3.0115 × 10²³
particles of CO₂ = 3.0115 × 10²³/6.023 × 10²³ = 0.50.

49. There are 10 protons and 17 neutrons in the nucleus of an element. What is its mass number? [RRB Group D 01/10/2018 (Evening)]

Correct Answer: (a) 27
Solution:

Mass number (A) = Number of protons (Z) + Number of neutrons (N).
According to the question,
Given Z = 10, N = 17, A = ?
∴ A = 10 + 17 = 27.

50. The atomic mass of aluminium is 27. The number of moles present in 81 grams of aluminium is______. [RRB Group D 01/10/2018 (Evening)]

Correct Answer: (d) 3
Solution:

Mass of substance = 81 g Mass of one mole (molar mass) of aluminium = 27 g
Number of moles = 𝑀𝑎𝑠𝑠 𝑜𝑓 𝑠𝑢𝑏𝑠𝑡𝑎𝑛𝑐𝑒/𝑀𝑎𝑠𝑠 𝑜𝑓 𝑜𝑛𝑒 𝑚𝑜𝑙𝑒
∴ Number of moles = 81/27 = 3.