Laws Of Chemical Combination

Question

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?

RankGuru · Accepted Answer

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.

Laws Of Chemical Combination

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