[Eva]

I have to calculate how much 0.05 M HCl I need to add too my puffer (pka=7.21).
nB=0.0503
pH= the start pH (I know that one from my experiment)
pH=pka+log(nB-x/nA-x)  <=> 7.21+log(0.0503-x/0.05-x)=6.8,x  = x= 0.0504

Is this correct?

[Borek]

my puffer

Oops, we are being read by kids.

(pka=7.21).
nB=0.0503
pH= the start pH (I know that one from my experiment)
pH=pka+log(nB-x/nA-x)  <=> 7.21+log(0.0503-x/0.05-x)=6.8,x  = x= 0.0504

You have not listed enough data to tell if it WOULD be correct, but this is for sure wrong. When you add acid to base nB gest converted to less nB (hence nB-x) but acid is being produced - hence nA+x.