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

Total Questions: 41

21. How many moles are present in 54 g of He ? [RRB Group D 31/10/2018 (Afternoon)]

Correct Answer: (b) 13.5 moles
Solution:

The number of  moles of Helium (He) in 54 g is : n = 𝑚/𝑀,
where m (mass of substance) = 54 g, M (molar mass of Helium) = 4 g/mol.
Mole (n) = 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑢𝑏𝑠𝑡𝑎𝑛𝑐𝑒/𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠
= 54/4 =  13.5 moles.

22. How many atoms of sulfur are present in 0.6 moles of SO₂? [RRB Group D 31/10/2018 (Evening)]

Correct Answer: (b) 3.613 × 10²² atoms
Solution:

23. What will be the mass of 0.6 mole atoms ? [RRB Group D 1/11/2018 (Evening)]

Correct Answer: (c) 0.6 g
Solution:

∵ Mole = 𝐺𝑖𝑣𝑒𝑛 𝑚𝑎𝑠𝑠/𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠
⇒ 0.6 = 𝐺𝑖𝑣𝑒𝑛 𝑚𝑎𝑠𝑠/1
(∵ Molar mass of H-atom is 1 g/mol)
⇒ Given mass = 0.6 × 1 = 0.6 g.

24. Given that the mass of 1 mole of oxygen atom is 16 g. What is the molecular weight of ozone ? [RRB Group D 5/11/2018 (Morning)]

Correct Answer: (b) 48 g/mol
Solution:

Number of oxygen atoms in ozone = 3.
Mass of one oxygen atom = 16 g/mol.
Molecular weight of ozone
= Number of atoms × Mass per atom
= 3 atoms/molecule × 16 g/mol/atom
= 48 g/mol.

25. The atomic mass of Aluminum is 27. The number of mole present in 54 g of Aluminum will be: [RRB Group D 5/11/2018 (Afternoon)]

Correct Answer: (a) 2
Solution:

Given : Molar mass of aluminium = 27 g,
Mass of aluminium = 54 g
∵ Number of moles
=  𝐺𝑖𝑣𝑒𝑛 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡/𝑀𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠 𝑜𝑓 𝑒𝑙𝑒𝑚𝑒𝑛𝑡 = 54/27 = 2.

26. Calculate the mass of 0.5 mole of N₂ gas. (mass to moles of molecule) [RRB Group D 15/11/2018 (Morning)]

Correct Answer: (a) 14 g
Solution:

Given that : Number of moles of N₂ gas = 0.5 mol,
Molar mass of N₂ = 28g.
∵ Number of moles (n) =  𝐺𝑖𝑣𝑒𝑛 𝑚𝑎𝑠𝑠/𝑀𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠
0.5 mole =  𝐺𝑖𝑣𝑒𝑛 𝑚𝑎𝑠𝑠/28 𝑔𝑟𝑎𝑚
Given mass = 0.5 × 28 = 14 g.

27. What is the maximum number of electrons that can be accommodated in the M shell of an atom? [RRB Group D 15/11/2018 (Afternoon)]

Correct Answer: (d) 18
Solution:

In the electron configuration of atoms, the M shell corresponds to the third principal energy level (n = 3). According to the formula 2n² , where n is the principal quantum number, the maximum number of electrons that can be accommodated in the M shell is 2 × 3² = 18.

28. What is the percentage of nitrogen in NH₄NO₃ ? [RRB Group D 3/12/2018 (Evening)]

Correct Answer: (a) 35%
Solution:

29. Write the number of neutrons, protons and electrons in ¹⁹ ₉F. [RRB Group D 3/12/2018 (Evening)]

Correct Answer: (c) 9 protons, 9 electrons, 10 neutrons
Solution:

Protons (Atomic Number) : 9 (since it’s fluorine, which has an atomic number of 9)
Electrons : Also 9 (since the number of electrons equals the number of protons in a neutral atom)
Neutrons = Mass Number (A) - Protons (Atomic Number) = 19 - 9 = 10.

30. How many atoms of sulfur are present in 0.6 molecules of SO₂ ? [RRB Group D 4/12/2018 (Evening)]

Correct Answer: (d) 3.613 × 10²³
Solution:

1 mole of SO₂ molecules = 6.022 10²³ × molecules.
0.6 mole of SO₂ molecules = 6.022 × 10²³
× 0.6 = 3.613  × 10²³ molecules.
Since, one molecule of SO2 contains 1 atom of Sulphur.
Therefore, 3.613 × 10²³ molecules of SO₂ contain
=  3.613  × 10²³ × 1 = 3.613  × 10²³ atoms.