chemistry : class12 Abundance of Oxygen in Earth’s Crust : Occurrence of Group 16 elements : p-Block Elements MCQ Questions and Answers
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Abundance of Oxygen in Earth’s Crust
MCQ questions and answers.
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MCQ Questions
1. Which statement most accurately describes the abundance of oxygen in the Earth's crust?
A.
Oxygen constitutes approximately 46.6% of the crust by mass and is its most abundant element
B.
Oxygen constitutes approximately 21% of the crust by mass and is its second-most abundant element
C.
Oxygen constitutes approximately 78% of the crust by mass and is mainly present in the free state
D.
Oxygen constitutes approximately 8.1% of the crust by mass and is less abundant than aluminium
Correct Answer:
Oxygen constitutes approximately 46.6% of the crust by mass and is its most abundant element
Explanation:
Oxygen makes up about 46.6% of the Earth's crust by mass, making it the most abundant element there. It occurs predominantly in combined forms such as oxides, silicates, carbonates, and other minerals.
2. A representative 2.50 × 10^3 kg sample of Earth's crust is assumed to have the standard oxygen abundance of 46.6% by mass. What mass of oxygen does the sample contain?
A.
1.165 × 10^2 kg
B.
1.165 × 10^3 kg
C.
5.360 × 10^2 kg
D.
2.034 × 10^3 kg
Correct Answer:
1.165 × 10^3 kg
Explanation:
The oxygen mass is 0.466 × 2.50 × 10^3 kg = 1.165 × 10^3 kg. The calculation uses the accepted mass abundance of oxygen in the Earth's crust.
3. If oxygen accounts for 46.6% and silicon for 27.7% of the Earth's crust by mass, what is the ratio of the mass of oxygen to the mass of silicon in an average crustal sample?
A.
0.594:1
B.
1.68:1
C.
2.33:1
D.
74.3:1
Correct Answer:
1.68:1
Explanation:
The mass ratio is calculated as 46.6/27.7 = 1.68. Thus, oxygen has about 1.68 times the mass abundance of silicon in the Earth's crust.
4. A crustal sample contains 46.6 g of oxygen for every 100 g of total sample. Which conclusion follows directly from this composition?
A.
Oxygen is the most abundant element in the sample by mass
B.
Oxygen is present only as molecular oxygen, O2
C.
The sample contains 46.6 mol of oxygen atoms per 100 g
D.
Oxygen is the second-most abundant element after silicon
Correct Answer:
Oxygen is the most abundant element in the sample by mass
Explanation:
A mass fraction of 46.6% is the accepted abundance of oxygen in the Earth's crust and is higher than that of silicon, aluminium, iron, and other major crustal elements. This value does not imply that oxygen occurs mainly as O2 or that its amount in moles is 46.6 mol.
5. The Earth's crust contains about 46.6% oxygen by mass, whereas the atmosphere contains about 21% oxygen by volume. Why should these two percentages not be directly treated as the same type of abundance?
A.
The crustal value is a mass percentage of a solid mixture, whereas the atmospheric value is a volume percentage of a gas mixture
B.
The crustal value is a volume percentage of solids, whereas the atmospheric value is a mass percentage of gases
C.
Both values are mass percentages, but the crustal value refers only to free oxygen
D.
Both values are volume percentages, but oxygen has a different atomic mass in the crust
Correct Answer:
The crustal value is a mass percentage of a solid mixture, whereas the atmospheric value is a volume percentage of a gas mixture
Explanation:
The approximately 46.6% figure refers to oxygen by mass in the Earth's crust, where oxygen is chiefly chemically combined. The approximately 21% atmospheric figure refers to the volume fraction of molecular oxygen gas.
6. If 40% of a 500 g crustal specimen is assumed to be oxygen-bearing material and oxygen constitutes 46.6% of that oxygen-bearing portion by mass, what mass of oxygen is present?
A.
23.3 g
B.
93.2 g
C.
200.0 g
D.
233.0 g
Correct Answer:
93.2 g
Explanation:
The oxygen mass is 500 g × 0.40 × 0.466 = 93.2 g. This applies the crustal oxygen mass fraction to the specified portion of the sample.
7. Which ordering correctly represents the approximate mass abundances of the four most abundant elements in the Earth's crust?
A.
Oxygen > silicon > aluminium > iron
B.
Silicon > oxygen > iron > aluminium
C.
Oxygen > aluminium > silicon > iron
D.
Silicon > oxygen > aluminium > iron
Correct Answer:
Oxygen > silicon > aluminium > iron
Explanation:
The approximate mass abundances are oxygen 46.6%, silicon 27.7%, aluminium 8.1%, and iron 5.0%. Therefore, oxygen is the most abundant among these elements.
8. A mineral-rich crustal portion has a total mass of 1.00 tonne. Using the standard oxygen abundance of 46.6% by mass, which statement is correct?
A.
It contains approximately 46.6 kg of oxygen
B.
It contains approximately 466 kg of oxygen
C.
It contains approximately 536 kg of oxygen
D.
It contains approximately 814 kg of oxygen
Correct Answer:
It contains approximately 466 kg of oxygen
Explanation:
One tonne equals 1000 kg. Therefore, the oxygen mass is 0.466 × 1000 kg = 466 kg.
9. Which interpretation of the approximately 46.6% oxygen abundance in the Earth's crust is scientifically appropriate?
A.
Nearly half the crustal mass is oxygen atoms chemically combined with other elements
B.
Nearly half the crustal volume is occupied by gaseous O2 molecules
C.
Oxygen contributes nearly half the crustal mass exclusively as water
D.
Nearly half the crustal mass consists of isolated oxygen atoms in cavities between minerals
Correct Answer:
Nearly half the crustal mass is oxygen atoms chemically combined with other elements
Explanation:
The crustal abundance is a mass percentage of oxygen atoms, which are mainly incorporated into solid compounds such as silicates and oxides. It does not indicate that free gaseous oxygen or water alone accounts for this fraction.
10. Suppose two equal-mass crustal samples contain oxygen abundances of 46.6% and 45.0%, respectively. What is the relative decrease in oxygen abundance from the first sample to the second?
A.
1.6%
B.
3.4%
C.
3.56%
D.
35.6%
Correct Answer:
3.4%
Explanation:
The absolute decrease is 46.6 − 45.0 = 1.6 percentage points. Relative to the first abundance, the decrease is (1.6/46.6) × 100 ≈ 3.4%.
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