[neon]

A 2.5 gm impure sample containing weak monoacidic base (Mol wt =45) is dissolved in 100 ml water and titrated with 0.5M HCl when 1/5 of the base was neutralised the pH was found to be 9 and at equivalent point pH of solution is 4.5. Find concentration of salt at equivalence point.

Let the initial concentration of base be c. When 1/5 of the base is neutralised amount of base left is equal to 4c/5 and concentration of salt formed is c/5.
Using Henderson's equation
pOH = pKb +log(1/4)

This gives me pKb = 5.6.
At equivalence point
Total amount of base is converted into salt. Thus concentration of salt is c.
Assuming hydrolysis of the salt
pH = 7-1/2pKb-log(c)

This gives me C=0.5. But the correct answer says that it should be 0.25.

[Borek]

pH = 7-1/2pKb-log(c)

Check it out.

[neon]

What's wrong with that equation? In salt hydrolysis of a salt containing weak base and strong acid,

[itex][H+] = \sqrt{\dfrac{K_w C}{K_b}} [/itex]

Converting the above into logarithms gives exactly the same expression I wrote earlier.

[Borek]

Converting the above into logarithms gives exactly the same expression I wrote earlier.

That's where you are wrong. Check your math.

[neon]

Oops! That was a silly mistake.  Thanks for pointing out.