Question

Given that the pKa of acetic acid is 4.8, what would be the likely result of adding 1 mL of 0.1 M HCl to a 0.1 M acetate buffer at a pH of 5.0?

A. The pH would drop slightly.
B. The pH would increase significantly.
C. The pH would drop significantly.
D. The pH would increase slightly.

Answer 1

any idea?

Answer 2

Well I feel like it might D maybe?

Answer 3

why did you choose that?

Answer 4

Since the pKa is 4.8

Answer 5

Actually, I think it's A

Answer 6

OK now a little bit closer, If you add an acid to a buffer always the pH is going to drop. If you add a base the pH will increase.
How do you know if the pH is dropping slightly or significantly?

Answer 7

It would drop significantly, since the pKa is 4.8?

Answer 8

The pKa is not going to tell you if the pH is going to drop slightly or significantly. The buffer capacity and the concentration of the buffer and the amount of acid that you add is going to tell you if the pH change slightly or significantly

Answer 9

Ohh okay! So the answer is C?

Answer 10

Shouldnt it be A? @Cuanchi Can u explain it a little more?

Answer 11

the problem is giving you two very similar options A and C, how can you figure out which one is the correct answer?

Answer 12

The pH?

Answer 13

you have the Henderson-Hasselbalch Equation
pH = pKa + log ([base]/[acid])

when the problem said that the concentration of the buffer is 0.1M that means that the concentration of the base/acid = 1 (both are 0.1M) an then the log 1 = 0 then the pH of the buffer is equal to the pKa.

when you add HCl to the buffer, the HCl is reacting with the sodium acetate , and produce more acetic acid and decrease the concentration of sodium acetate in the buffer.
At this point the relationship [base]/[acid] is <1 then the log of a number <1 is negative and in the equation the pH is going to be lower than the pKa

Answer 14

I thought it in this way: since its an acid there should be a slight drop in pH as its concentration is small

Answer 15

@Cuanchi Is it C then?

Answer 16

Your idea @Rushwr is correct the only thing you have to be sure that the amount of strong acid (moles) is lower than the amount of salt that you have in your buffer. If the amount of strong acid is bigger than the amount of salt you don't have buffer anymore and you have just acid and NaCl

Answer 17

I will go for A because the problem doesn't mention the volume of the buffer that you have

Answer 18

yey my answer is correct ! @Cuanchi
Thank you