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Question

Assertion (A) : BF3 is a weaker Lewis acid than BCl3.
Reason (R) : BF3 is is less electron-deficient than BCl3.

A
(A) is correct; (R) is correct and (R) correctly explains (A).
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B
(A) is correct; (R) is correct and (R) doesn't explain (A) correctly.
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C
(A) is correct; (R) is incorrect.
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D
(A) is incorrect; (R) is correct.
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Solution

The correct option is A (A) is correct; (R) is correct and (R) correctly explains (A).
BF3 is a weaker Lewis than BCl3 because of the more effective back-bonding in boron trifluoride. B and F belong to the same period and this the back-bonding (donation of a lone pair on F into the pure, unhybridized 2p orbital of B. As for BCl3, the back-bonding is not as effective because of the difference in energies of the pure 2p orbital of B and the orbital occupied by the lone pairs of Cl. Thus, BF3 is a weaker Lewis acid.

Thus, both (A) and (R) are correct and the latter accurately explains why (A) is true!

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