[Lbllaz]

A 18mL 0.1M of acetic acid was added to 2mL 0.1M sodium acetate create a buffer. Pka is 4,74 . I need to find the pH after the addition of 0.1M 10mL HCl . i Really dont know how to go about solving this.  Thank you !

[Babcock_Hall]

It is a Forum Rule (see red link above) that you must show your attempt before we can help you.  However, I will give you a small hint to get started.  You will need to use the Henderson-Hasselbalch equation to solve the problem completely.  I suggest starting with the situation before the addition of HCl, inasmuch as it is simpler.

[Lbllaz]

Sorry didn't see that part in the rules.
I used the Henderson-Hasselbalch to find the pH of the buffer before the addition of HCl which resulted in 3.78.
Calculations :
c1v1=c2v2 => c2(acid)=(0.1*18)/20 = 0.09 -  c2(acid)=(0.1*2)/20 = 0.01
pH= 4.74 + log(0.01/0.09)
pH=3.78

Am stuck in the part with the addition of HCl of 10mL 0.1M , as the reaction will have no more base left (i assume) . I have tried multiple ways of solving it like
10^-3.78 = 1.659*10^-5
=> pH= -log√(1.659*10^-5 *((18/1000*0.1)/0.03)) = 2.5 , but my tutor told me it had to be 3 point something so i must be doing something wrong and i kinda dont know what to do now.

[AWK]

You have an excess of HCl. Neglect acetic acid and NaCl - calculate pH of the solution of the rest of HCl