일단 pH of buffer 라는 것은 

그 양이 얼마가 됬든 

ratio of weak acid and conjugate base 가 정합니다 

공식이 pH = pKa + log[A-]/[HA] 이지요?

pKa 야 constant 일테니 

A- concentration 과 HA concentration 의 비율이 pH 를 정합니다 

즉 0.1M/0.2M 이든 1M/2M 이든 숫자가 달라도 

conjugate base 와 weak acid 의 비율이 같다면 pH 는 같습니다 

그러나 표에서 보듯이 

pH value after addition of either strong acid or strong base 

는 달라집니다 

buffer 1의 pH 가 많이 바뀌었는데 

그건 buffer 의 양 이 더 적을 수도 있고 - 지금처럼 volume 이 같은 경우라면

buffer 의 concentration 이 더 적은 경우입니다 

이부분을 개념강의에서도 설명했는데요 

이것을 buffer capacity 라고 부릅니다 

lower concentration of buffer has a weaker capacity to resist a change in pH 인 것이죠 

아주 중요한 개념이니 꼭 알고 있어야 합니다 

for the equation you wrote for the dissociation of weak acid is 

little different from the question, since lactic acid is not used for the buffer in the question, 

it is the buffer that consists of acetic acid and acetate. 

Anyways, for the dissociation of acetic acid or the equilibrium equation of that buffer will be given as 

HC2H3O2 + H2O --> H3O+ + C2H3O2- 

or shortly, 

HC2H3O2 --> H+ + C2H3O2- 

For the reaction of how a buffer reacts with HCl added, 

the conjugate base, C2H3O2-, will react with additional HCl,

C2H3O2- + HCl --> HC2H3O2 + Cl-

but Cl- is also a spectator, then it can be eliminated from the equation too, 

C2H3O2- + H+ --> HC2H3O2 

this is the net ionic equation of the neutralization. 

So, those two reactions given above are quite similar, but just written in an opposite direction. 

However, they have totally different meanings, so you should be familiar with all of these equations. 

I explained about how you set up the equations and calculate its pH for each case in concept lecture, 

so i definitely recommend you to watch carefully those concept lectures a few more times repetedly. It will definitely help you! 

Hope it helps!

Best regards, 

Kelda