How do I calculate change in pH in a buffer?

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) %26lt;=%26gt; 7.21+log(0.0503-x/0.05-x)=6.8,x = x= 0.0504



Is this correct?How do I calculate change in pH in a buffer?
I don't think it is, but I'm not certain what you're trying to do. You appear to be preparing a buffer at pH=6.8 by adding HCl to the conjugate base of a weak acid. It's not clear though whether nB is a number of moles, or concentration, or something else.



If nB is the number of moles of base, then by adding a volume x of acid you add (0.05 M)x moles of acid. Since base and acid are dissolved in the same total volume V,

[base]/[acid] = { (0.0503 mol)/V } / { (0.05x mol) / V } = (0.0503) / (0.05x)



and x = 2.6 L (!).



If nB is the concentration of base and not the moles of base, then the amount of base is

nB * Vb where Vb is the volume of base you start with, and now

[base]/[acid] = (nB * Vb) / 0.05x



Now you can solve for (x / Vb), the volume of acid over the volume of base:

x / Vb = 2.6 mL/mL



which is much more reasonable.