Which is more acidic Me3C-OH Or Me3Si-OH and why ?

Me3Si-OH is more acidic than Me3C-OH because after removal of H+ it's negative ions (Me₃Si-O-)stabilized by 2pπ—3dπ back bonding. While Me3C-O- is unstable than Me3C-OH.

B-F bond length in BF3 is shorter than B-F bond length in (BF4)- why?

B-F bond length increases when BF₃(130 pm) reacts with F- to form (BF₄)- [143 pm]. Its due to absence of Back-bonding in (BF₄)- hence B-F bond has completely single bond character

Dipole moment of PCl3F2 and P(CH3)3(CF)2 molecules are zero while dipole moment of PCl2F3 and P(CH3)2(CF3)3 are non zero why?

According to bent rule more electronegative atom or group attached those hybrids orbital have minimum S- character.

There is in trigonal bipiramidal (TBP) Geometry we known that axial orbital hare no s-character so F and -CF₃ group are attached with equatorial positions.  Hence dipole moment of P(CH₃)₂(CF₃)₂  is zero. While in case of PCl₂F₃ and P(CH₃)₂(CF)₃ molecules one of the F and –CF₃ group are also present at equatorial position hence there is net dipole moment.

Related Questions:

What is Bent’s rule of hybridization?

Which of the following compound have longest (S=O)bond length , O=SF2, O=SCl2, O=SBr2.

Why Bond length of O-O is greater in H2O2 than O2F2?

How to arrange increasing (C-H) bond length in increasing order and H-C-F bond angle in the given compounds, CH4, CH3F, CH2F2 and CHF3 ?

Dipole moment of PCl3F2 molecule is zero while dipole moment of PCl2F3 molecule is non zero why?

Dipole moment of P(CH3)2(CF3)2 molecule is zero while dipole moment of P(CH3)2(CF)3 molecule is non zero why?