chemistry about base and acid and buffer

Question

hello, I was wondering if you'd be able to help me with chemistry?
tomorrow I have an exam and I don't understand some things.

1. when can you say of a buffer is effective or not?
2. so the prof gave us pH=12 , it is a metalhydroxide and we have to calculate Ks and S. Any idea? it was an exercice in the other exam but I forgot the whole question, i just remember the essential.
3. I don't understand very well what a amfiprotic is? i know it's compound that act like an acid and base. But how can we regognize them?

Answer 1

1. when can you say of a buffer is effective or not?  If you add small amounts of strong acid or base, and the pH does not significantly change, then you have an effective buffer.  But is this the real question?  Or is it maybe really asking how to make an effective buffer? 

2. so the prof gave us pH=12 , it is a metalhydroxide and we have to calculate Ks and S. Any idea? it was an exercice in the other exam but I forgot the whole question, i just remember the essential.

pH = 12 should tell you how to find the [OH^-].  Write the Solubility equation for M(OH)₂ ↔ M²⁺  + 2OH^- (aq)

S is the molar solubility, and for every mole of the metal hydroxide that dissolves, you get one mole of the metal and 2 moles of OH. 

M(OH)₂ ↔ M²⁺ + 2OH- (aq)

                S         2S

 

Ksp = [M²⁺][OH^-]² = (S) (2S)² = 4S³

The Ksp expression in terms of "S" is therefore Ksp = 4S³, so if you have the Ksp you should be able to find S.  In this case, you don't have the Ksp but you do have the OH^- concentration (via the pH value). The OH - concentration is "2S", so you should easily be able to find S. Can you solve it from there or do you also not know how to find OH- from pH?

3. I don't understand very well what a amphiprotic is? i know it's compound that act like an acid and base. But how can we recognize them? Usually you are talking about something that can either donate an H+ or accept one.  Water is a classic example, as H₂O can gain an H⁺ and become H₃O⁺ or it can lose one and become OH-.  Conjugate bases of polyprotic acids are also amphoteric or amphiprotic.  For example, H2SO4 can be an acid.  HSO4-, its conjugate base, can still act as an acid because it has another H+ it can donate.  But it is the conjugate base of H2SO4, so it can also act like a base!  (but a very weak one since H2SO4 is a strong acid). 

 

Hope that helps :)

Answer 2

Hi Chaimae, 

 

Base is essentially a salt with "OH" such as NaOH or sodium hydroxide, KOH or potassium hydroxide and so on.

 

Acid is an acid with only "H"  and no "O" that can accept another "H" ion, such as HCl or hydrochloric acid. 

 

pH of an acidic solution is always lower than 7 (at pH &, solution is neutral, neither acidic or basic).

 

 NaOH or sodium hydroxide and KOH or potassium hydroxide are usually added to an acidic solution to neutralize it to and elevate its pH close to 7 or standard buffer.

 

Likewise HCl or hydrochloric acid is added to bring down the pH of a basic solution to a neutral buffer of 7 or close to it. 

 

Please refer further to your text book and PowerPoint from lecture and post again with correct spelling and specific question, I shall try to help. 

 

Best,