Thanks! But may i ask how did u get the conc for BH+ and B?

For which part?

The first part, first option

No. Of moles of B is the number of moles of HCl required to neutralise it. Then from number of moles you divide by the volume of B initially you get [B]. This of course, we are assuming the amount of B dissociated is negligible compared to its initial concentration. So [B] before dissociation = [B] after dissociation. An analogy would be if you got a million dollars, I take 1 dollar from you, it's not going to affect your financial status since 999,999 and 1,000,000 is about the same.

Then based on the dissociation equation, the OH- and BH+ are equimolar. So [OH-] = [BH+]

So intital conc of B equals to conc after dissociation is a necessary assumption we must make?

Most of the cases when you have to assume.

But since this case you know [OH-], you can use ICE table to find out final concentration of base after dissociation. Then find out kb. the answer won't deviate too much from 8 e-04.

Ohh i see thank you!

Remember when you assume, it's good to check back that the dissociation is less than 5% of the original concentration.

If I'm not wrong, all the time it should be. It is like to tell the examiner, "hey, I know that the dissociation is very small compared to the original conc. that's why I can assume dissociation is negligible (since it's <5% of original conc.)

Yup okay! Btw, for option B i tot conc of H+ is just 0.01x10x10^-3? Why do we need to divide by55?