[titration]

In this exercise I have to find out how much acid I need to add for my solution to go to 7.
The acid is 0,1 M HCl.
The solution I have to add the acid to is 0,2M, 250mL, with a pKb of 7,6.
In the exercise prior to this we found out at pH 7 the solution would be: 20 % acid form, 80 % baseform.
My idea for solving it is the following but I'm not sure this is allowed:
At pH of the solution without acid >1 % is on the acid form, so this is neglected.
At pH 7:
20%*n(solution)=n(acid)
20%*(250 mL*0,2M)=0,1M*V(acid)   => Vacid= 100 mL
Anybody who can help me?

[Borek]

Do you know Henderson-Hasselbalch equation?

Assume - when adding strong acid - conjugate base of the buffer is quantitatively protonated.

[titration]

Yes it is:
pOH=pKb+log([Acid]/[Base])
Is this then correct?:
n(base)=0,2M*0,25L=0,05mol
therefore base equals 0,05mol-[Acid]
then:
7=7,6+log(acid/(0,05mol-acid) => [Acid]=0,010038 moles
n/C=V
0,010038 moles / 0,1M = 100,38 mL?

[Borek]

Result looks OK.