Question

How can a molecule such as bcl3 act as an acid if it has no protons to donate?

Answer 1

BCl₃ is Lewis acid, that is an acceptor of electronic couple. Borum accepts electronic couple on the emptiest orbital.

BCl₃ + HCl = H[BCl₄]

BCl₃   +   Cl⁻   =   [BCl₄]⁻
Lewis    base
acid