# Henderson Hassalbach equation problem

1. Apr 17, 2010

### hianghao

1. The problem statement, all variables and given/known data
I am trying to prepare a 1L Tris-HCl buffer 1M pH7. The pKa value is 8.06. The question is, how many molar of HCl should be added to produce this buffer?

2. Relevant equations
pH = pKa + log [base/acid]

3. The attempt at a solution
7=8.06 + lg (a/b)
-1.06=lg (a/b)
a/b = 0.0871 -----1
a+b= 1M
a=1-b --------2
Hence,
(1-b)/b = 0.0871
b = 0.92M
a = 0.08M
So, the molarity of HCl to be added is 0.92M or 0.08M. Why?

2. Apr 17, 2010

### Staff: Mentor

What reaction takes place when you add HCl?

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3. Apr 17, 2010

### hianghao

C4H11NO3 + HCl <-> C4H10NO2Cl + H2O?
Is this correct?
Hence HCl to be added is 0.92M? Because b=salts conc while a=acid/base conc?

4. Apr 18, 2010

### Staff: Mentor

No. HCl is neutralized, but there is no water between products. TRIS hydrochloride is produced.

Buffer is a mixture of salt & acid/base, but acid and conjugate base.

You need to identify an acid and its conjugate base in the case of TRIS.

What is a Broensted acid? What is a Broensted base? How are they related?

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methods

5. Apr 18, 2010

### hianghao

So buffer should be acid+conjugate base or base + conjugate acid.
Bronsted Acid donate proton while base receive proton.
HCl is suppose to be the acid cuz it has H+ to be donated.

If no water product, could it be
(CH2OH)3CNH2 + HCl <> (CH2OH)3CCl + NH3?
but still it doesn't become TRIS hydrochloride..
Or it becomes (CH2OH)2CClNH2+ CH3OH? I can't hink of other possiblity..

Last edited: Apr 18, 2010