Creating a buffer out of 0.100M solutions

[JwdePhysics]

Homework Statement

The questions asks which of the following would make a buffer given that all solutions are 0.100M and you have solutions of acetic acid, sodium hydroxide, and hydrochloric acid.

1. 20.00mL of each solution is mixed.
2. 20.00mL of acetic acid and 30.00mL of NaOH are mixed
3. 30.00mL of acetic acid and 20.00mL of NaOH are mixed
4. 20.00mL of acetic acid and 20.00mL of NaOH and 10.00mL of HCL are mixed.

Homework Equations

None necessary

The Attempt at a Solution

So I know that 2. is wrong because once all the acetic acid and NaOH are mixed, there will be 10 mL left of a strong base which is not a buffer. I know 3 is right because 30.00mL of acetic acid added to 20.00mL of NaOH will leave 10.00mL plus some base when the NaOH reacts so that creates a buffer. However I am not sure how to solve 1 and 4 with 3 solutions each. Does anyone have any advice for determining if 1 and 4 are right?

 

[Borek]

Buffer needs both weak acid and its conjugate base to be present. Will they be present in 1. and 2.?