Question

BF3 is a weaker lewis acid than BCl3

Answer 1

In contrast, toward weakbases such as CO, BF3 is a stronger Lewis acid than BCl3. ... In contrast, in the formation of a complex with a weak base such as CO, the BX3 is barely distorted from planarity and so the acidity of BF3 is greater than that ofBCl3 because the charge on boron is greater in BF3 than BCl3.

Answer 2

because f higly electronegative but cl is less elctro ngtv then f so doesn't get positive ion ... so its is weakr lewis acid than bcl3