24. BF3 does not have proton but still acts as an acid and reacts with $\stackrel{..}{\mathrm{N}}{\mathrm{H}}_{3}$. Why is it so? What type of bond is formed between the two?

BF3 is an electron deficient compound and hence acts as Lewis acid. $\stackrel{..}{\mathrm{N}}{\mathrm{H}}_{3}$ has one lone pair which it can donate to BF3 and form a coordinate bond. Hence, NH3 acts as a Lewis base.
${\mathrm{H}}_{3}\mathrm{N}:\to {\mathrm{BF}}_{3}$