[timchai]

ans said .A Bronsted-Lowry base is a proton acceptor.
H2O does not self ionize appreciably, while HF does.
OH- can thus accept a proton better than F-, and hence OH- is a
stronger Bronsted-Lowry base than

but i want to ask why . i think the ans is not correct, i think the ans is OH- more stronger than F- , bcoz of electronegativity , so tendency of accept electrons greater

[BluRay]

ans said .A Bronsted-Lowry base is a proton acceptor.
H2O does not self ionize appreciably, while HF does.
OH- can thus accept a proton better than F-, and hence OH- is a
stronger Bronsted-Lowry base than

but i want to ask why . i think the ans is not correct, i think the ans is OH- more stronger than F- , bcoz of electronegativity , so tendency of accept electrons greater

In case, it would be the tendency to "give" electrons (to form a covalent bond) which is greater for OH^-. However the first explanation is actually better: you have to compare the equilibrium reactions:

OH^- + H₂O <-->   H₂O + OH^-

F^- + H₂O  <-->   HF + OH^-

The first reaction has the greater equilibrium constant in water solution and at room T and it's for this reason that OH^- is a stronger base than F^-

[spirochete]

BluRay's equilibrium description is correct but I'll add some qualitative explanation.

Bronsted-Lowry basicity is the tendency of a molecule to accept a proton, not an electron.  It's 100% true that F- is a weaker base than OH- because of electronegativity. 

Having a negative charge is destabilizing, but a more electronegative atom is destabilized less by having a negative charge.  In other words, a more electronegative atom is "happier" with a negative charge.  With neutral bases there is simply an electron pair but the same logic applies.

Acquiring a proton means a base no longer has full possession of an electron pair.  More electronegative atoms will be less likely to do this because they are relatively stable with a lone pair and/or negative charge.

[BluRay]

It's 100% true that F- is a weaker base than OH- because of electronegativity. 

Then why I^- (for example) is not a stronger base than F^- or than OH^- in water solution? Iodine is less electronegative than Fluorine or Oxygen.