Which of the following combinations would act as buffered solutions

[JessicaHelena]

Homework Statement

Which of the following combinations would act as buffered solutions?
a. HCl and NaCl
b. CH_3COOH and KCH_3COO
c. H_2S and NaHS
d. H_2S and Na_2S

Homework Equations

none really... The acid needs to be weak and the base strong though.

The Attempt at a Solution

HCl is one of the strongest acid, so that eliminates a. Since the conjugate base of H_2S should be HS^-, that also eliminates d., which has somehow gotten rid of the other H as well. However, I'm having a difficulty choosing between b and c because CH_3COOH and H_2S are both weak acids (from what I've learnt), which makes them good candidates for a buffered solution.

 

[Borek]

The acid needs to be weak and the base strong

No, that's not how it works. Buffer needs presence of an acid and its conjugate base - neither should be really strong. Plus, pKa + pKb = pKw, so their strengths are not independent.

Somehow you managed to get close to the correct answer though - both b and c will work as buffers.

 

[JessicaHelena]

Oh okay. However, since this is a multiple choice question, I think I’m supposed to choose only one of them...

 

[Borek]

If it is a multiple choice you can as well select two.

If you were told there is only one correct answer one of these solutions won't be stable and will be loosing (quite fast) one of its components.

 

[JessicaHelena]

Is there any way I can tell the speed of the reactions by looking at the chemical formulas?

 

[Borek]

No, but it is not about speed of the reaction. What do you know about properties of H₂S?