Laws Of Chemical Combination
A compound is formed by elements A and B such that two atoms of A combine with three atoms of B. If the atomic mass of A is 20 and that of B is 16, what is the percentage composition of A in the compound?
Select the correct option:
Solution
45.45%
Percentage composition is defined as the mass contribution of each element divided by the total molar mass of the compound, multiplied by 100. The compound has the formula (A_2B_3). The molar mass of the compound is (2 \times 20 + 3 \times 16 = 40 + 48 = 88 \text{ g mol}^{-1}). The mass contribution of A is (2 \times 20 = 40) g. Therefore, percentage of A (= (40/88) \times 100 = 45.45%). Option 40.00% is incorrect because it represents just the mass of A (40 g) expressed as a whole number, ignoring the total molar mass of 88 g. Option 54.55% is the percentage of B, not A, obtained as ((48/88) \times 100). Option 62.50% does not correspond to any valid calculation with the given atomic masses. This follows directly from the NCERT treatment of percentage composition in the chapter on basic concepts. Plausibility check: the percentages of A and B must sum to 100%, and (45.45 + 54.55 = 100%), confirming internal consistency.
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About This Question
- Subject
- chemistry
- Chapter
- some basic concepts in chemistry
- Topic
- laws of chemical combination
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
45.45%
Percentage composition is defined as the mass contribution of each element divided by the total molar mass of the compound, multiplied by 100. The compound has the formula (A_2B_3). The molar mass of the compound is (2 \times 20 + 3 \times 16 = 40 + 48 = 88 \text{ g mol}^{-1}). The mass contribution of A is (2 \times 20 = 40) g. Therefore, percentage of A (= (40/88) \times 100 = 45.45%). Option 40.00% is incorrect because it represents just the mass of A (40 g) expressed as a whole number, ignoring the total molar mass of 88 g. Option 54.55% is the percentage of B, not A, obtained as ((48/88) \times 100). Option 62.50% does not correspond to any valid calculation with the given atomic masses. This follows directly from the NCERT treatment of percentage composition in the chapter on basic concepts. Plausibility check: the percentages of A and B must sum to 100%, and (45.45 + 54.55 = 100%), confirming internal consistency.
This easy difficulty chemistry question is from the chapter some basic concepts in chemistry, covering the topic of laws of chemical combination. It appeared in the 2025 exam.
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